kicker zx2500 going into protect

Well I was driving around my area full tilt today no problems. But when i took my 20 min drive to work after about 15 min of playing full tilt it starts going into protect. Only when its loud though. Anyone know what could be the problem? I double checked all my wiring im pretty sure its all right the fact that it only goes into protect at the loudest volume. Also when i left work i had it full tilt for about 10 min until it started going into protect My thinkings are though Could my rca's put out a bad signal? I am only running 4g to my ground could that not be effecient enough? I dont have my wired fused coming off the second battery but the distance is very short could that be the problem? Also earlier i did accidently put the power wire into the ground input slot. Could my voltage be dropping to much? I cant figure out whys its doing this. If my wiring was bad wouldnt mess up at low volumes to? It only goes into protect whens its up just about all the way. But not full on protect just the green kinda fades into the red then back to green then fades into red

Sounds to me like you are overextending the sub.  The voice coil is starting to short out.  Eventually the sub will probably just quit.  The shorting of the voice coil is putting a wierd load on the amp and causing it to go into protect.