why does my horses hind legs give away?

I would telephone your vet AS SOON AS POSSIBLE!   This does not sound good.  

If a horse is not in condition, it will not be up to a long ride. Just like humans, a horse has to be conditioned and built up to the exercise program planned. I don't know what a 20k trail ride is, but it sounds long.

If a horse has been pastured for 2 years, the horse will need to be slowly built up in muscle and endurance. Even a horse who is ridden daily, will need to work up to a long ride.

I would call the vet right away in case your horse needs electrolytes and/or special treatment for his overall health and well-being.

It is assumed if a horse's legs or hind-quarters (or anything else) is giving out, you would STOP the ride and take him back to rest. He is not a robot, but a living creature. Call the vet before anything else gives out on this animal who was simply trying to please you.

Believe it or not horses are not designed to have humans riding on their backs and bouncing up and down on their spines.  Your hips might give out too if you have some self-centered creature riding on your back all day.  Try giving the horse a break and stay off his back for a couple of months maybe six months.  Personally I'd rather that you set the thing free and you not assume you have a right to possess it at all but I know that's not going to happen.  So give the damn horse a break and a vacation from towing your azz around all day.  maybe the horse will perform better if you give it a little R&R, let it hang out with some other horses, play don't ride it for a long time, pamper it a little bit, let him get some sun let him go swimming, and take it to the horse spa.  The horse is a living creature and riding you around on its back all day is not it's intended godly purpose. No matter what the medical problem is I think a vacation would help the horse greatly.

Think about your boss working you to death day after day and never giving you a vacation or break. The horse's body is telling you that it needs a vacation. The problem is most likely you. The horse is not a vehicle it's a living creature that needs the same kind of TLC you do which includes weekends off, vacations r&r socialization, play etc. 

Not even your car can just perform day after day without some TLC. Youre expecting a horse to be a vehicle instead of a living creature and it's body is starting to send signals that you're ignoring. The damn horse needs a break, along break.  

There may be some underlying medical condition but I think the first thing you need to do is stay off the damn horse for about 6 months, give it a break. God didn't design it so you can ride it. You riding no damn horse all day it's not part of its designed  purpose.  You're probably throwing all of its body functions out of alignment just like when a human has an injury it throws off your entire body balance. So believe it or not you riding and it's not part of its natural design purpose. Get the horse some chiropractic care, let it go swimming, let it roam free for a while let it socialize with other horses if you can. Give the damn thing a break. Let the horse get laid not as a breeder. Let the horse go flirt with some other horses maybe the horses just mentally tired of carrying your azz around and it's manifesting itself as physical problems. 

You think the horse is a car where you just patch it up jump back in it and drive. It's not a car it's a living creature therefore it needs much more such as TLC and a vacation.  The horse is a living creature with emotions, feelings needs for socialization and things like that. And it sounds like the horse is getting old maybe it's time to retire the horse.

Diagnosis: The horse has humanitis ridingmetodeathous syndrome. 

This horse needs to be seen by a VET RIGHT AWAY and tested for both EPM and Lyme disease, either of which can cause the kind of symptoms you describe. And while the vet is at it, have him or her run a LIVER PANEL on the horse, because one of the signs of liver disease (including cancer) is weight loss and weakness in the hind end, including but not limited to tremors and staggering. I add in the warning about liver disease because I lost my competition horse to liver cancer last summer, and neither I nor the vet nor anyone else had any idea what we were seeing until it was far too late to do anything about it.

If the horse checks out medically, you should have him evaluated for a slipped stifle. That can also cause the issues with "collapsing" behind that you describe.

Addition:

Macaria - The horse I spoke of that went over on top of me was the same.  Never a symptom before, and just fine after his first episode.  I had him vetted extensively, and they found nothing.   He was finally diagnosed with EPM quite some time later, when the protozoa had a chance to multiply and did enough damage it could be seen and diagnosed.

Neurological conditions tend to be progressive.  That means they get worse over time, in unpredictable times and ways.  Your horse looked perfectly healthy when the vets saw him.   But he's not. 

If you won't stop riding him, please at least pay close, close attention to any possible issues when you are riding him.  It might save your life.

Just one more thought...  a horse that's been in a paddock, unridden, for two years probably needs more time to build strength and stamina for an endurance ride.  It's conceivable that this is due to a lack of conditioning... in which case you're at fault and hurting him.

Original answer:

Just because the problem is intermittent now doesn't mean it's not a problem.  I've seen this happen with several horses, all of whom were diagnosed with severe neurological issues.

This is a BIG deal, and a chiro is not the right one to deal with it.  Get a vet out, get the horse diagnosed, and deal with it appropriately.

Do not ride the horse until you have done so.  You're endangering both the horse and yourself.

One of my horses that had a similar issue lost control of his hind end and went over backwards on top of me.  Needless to say, that was no fun at all.  Hospitals never are.

This could be anything from stroke to EPM to West Nile to Equine Encephalitis to...  Since he's a quarter horse it could also be HYPP or EPSSM.  It could also be something simple and easy to fix.  You won't know till you get the vet out to do testing and diagnose it.

You need to first of all not ride this horse until the issue is corrected (if it can be). This is extremely unsafe. Then, you need to get your veterinarian out. If both rear legs are giving out, it's likely a neurological issue or a problem with his spine. It may or may not be treatable. 

This is beyond the advice of strangers on the Internet, and may cause more issues for your horse. Please consult a vet, I'm surprised your chiropractor agreed to treat the horse before any vet consultation (to get an idea of why his hip was 'out' if anything).