Well, hopefully the amplifier was wired correctly. But either way there are so many factors that can cause this problem... But the biggest one is the amplifier must have been sending some distortion to your subwoofers which a subwoofer can take a certain level of distortion and be fine but too much will blow any subwoofer even the highest of high end subs at a high volume... when your subs hit the point of blowing they will tend to just blow out one at a time... they don't just pop both right away. I'd say get the sub that isn't blown off off of the amp before it blows too... distortion is clipping most of the time... when you destroy the subs it will eventually work it's way to the next component in the system which is your amp and then you will be out an amplifier and subs. save yourself the headache.......