First off, those subs are rated at 1100W and that amp puts out 1800W @ 1 ohm. That kind of current draw is probably taxing you battery and alternator right off the bat. Add air conditioning, headlights, etc and you are probably pulling more current from the battery than your amplifier can replenish. A high output alternator and a better battery could relieve that problem.
A second reason could be the guys that installed the system. Never underestimate the stupidity of the installers(the few that give bad reps to the rest of us). It might be something as simple as them hooking up the system using a constant 12v line to turn(and leave) the amps on.
I'd go with the first but it never hurts to check to see if the amps are still on when the vehicle is parked without the keys. A cheap five minute fix is better than spending an additional $300 for a electrical system upgrade(even though you still might need it).
Igitur qui desiderat pacem, praeparet bellum -- Vegetius
"I told her about my conversation three years ago with the political leader of Islamic Jihad in Gaza. "What's the difference between suicide, which the Koran condemns, and martyrdom?" I asked. "Suicide," he replied, "is done out of despair. But remember: most of our martyrs today were very successful in their earthly lives." In short, there was a future to live for--and they detonated it anyway."