Air tight ATM 211 and MAXX

I have currently encountered a shocking and dreadful experience with Air tight ATM 211 mono amps. I did not unplug the amps for 5 or 6 days from AC outlet and after 6 days when I turned on the power amps in a few seconds a very loud and shocking noise appeared in the speakers which I had to rush and turn off the amps before they damage the speakers. The noise was so loud and striking that I thought I had burnt the speakers! What could be the reason and how can I avoid such problem.
Thanks for your advise.