For what insane reason did Queen Victoria leave someone a pair of he knickers after he death ?

Probably thought the fabric could be reused to make sails for a boat or a giant circus tent.

Royals often gave workers pieces of clothing.

She didn't, after she died, they were lifted and fenced, if you'll pardon the expression.

That didn't happen.   Over time, people seem to think 'stealing' clothing worn by the royals might be a good investment.   As was the case here!    Frankly I question the buyer of such an item!!

She did'nt, they came into the possession of someone who worked for her and they handed them down in the family.

Weirdos steal panties. You don't even have to be The Queen. It's totally insane. If they're in an obvious place and unsupervised workmen are around, don't be surprised if a few go missing. If I were The Queen I'd have a fire going in my dressing room whenever it's changing time and kick them directly onto the flames. 

One of the servants kept it when she died.

She didn't, a pair of bloomers were stolen and others found and auctioned off, the buyer of them must have been insane, they sold for £thousands just because they were spread across the royal butt.