Why is my gartner snakes stripes turning red?

Because its getting older. Many species change color as they get older. With some snake species, even the patterns change. Juvenile yellow rat snakes, racers, indigos, and many other species are blotched or saddled, but when they get older, the markings and colors change. With most species of garter (not all), their dominant color is brown, green, dark green, or black, and the stripes are usually white, yellow, or light green. As they get older, both the dominant colors and the stripes can change in hues or colors. Here is a great site for garter snakes too show variations in color.

http://www.gartersnake.co.uk/photographs.htm

EDIT. Wow, the color is really changing quickly. Are the stripes actually "red" or just appear to be "turning" red? Meaning to say is it an off-red or orange? Has his eyes clouded over? Are the other colors changing hues? It sounds like you snake might be molting (preparing to shed). Take a look at it's eyes and belly and look for dullness in the overall color. Just before a snake sheds, a fluid is released from the undelying new skin to loosen up a layer of old skin. This cause rapid color variation. It's difficult to imagine red stripes as a result of molting, but possibly off-red, orange, or copper-brown. This is definitely not a sign of illness, malnutrition, or dehydration.

Anyway, if your snake is preparing to shed, the colors will return to almost normal for a couple of days, then it will shed within a day or two and be brighter than before. If this is the case, you will know in say, about a week. Just in case, don't handle it or feed it right now. It will not eat and handling can be harmful to the natural shedding process.

Probably because you have a red sided garter snake.