Riding the brakes (aka using the brake pedal as a foot rest for your left foot), frequent hard braking, sticking brake calipers, etc. can cause warped rotors. They can also warp if they're cheap quality. 

I've never even heard of rotors warping that fast whoever put them on is an idiot and clearly F'd up the install. Rotors should last for years upon years not months UNLESS you or the shop bought super cheap china made rotors then all bets are off. Either way you need to take it back to the installer and demand your money back and take it to someone who knows what the hell they are doing and what parts to buy because they clearly either installed them wrong or bought cheap crap parts or both. But just to be sure take it somewhere else first and have them asses the issue. It could be something else causing the problem but if you have another shop look and they say its incorrectly installed then at least its not your word vs the crap installer and you have proof incase the previous installer tries to worm their way out of making it right. 

junk rotors, stuck brakes, or improperly torqued lug nuts