Because they saw someone doing it and want to emulate them: they think it makes them look cool.
Because safety gear covering the face won;t allow the bill to face forward.
Because when they lean forward to gt close a closer look at something the bill gets in the way.
Their hair is messed up and they want to wear a hat indoors, but don't need it for the sun so it could be pointed in any direction. Whatever looks best on them or is most comfortable. I wear a hat backwards around my house and find it more comfortable than with the bill to the front.
Protect your neck from the sun.
Increase your peripheral vision. Sometimes you have to look side to side and you're already wearing a cap but there's nowhere to put it, so you just turn it backwards.