Amp get too hot, intermitant ground, or just bad connection somewhere 9pinched or pinned wire). The speaker connections can also be fine, but running at too low of an OHm load, which works the amp too hard and caused it too shut off.
Try disconnecting speaker wires from amp to test if there is a problem with the speaker wire or possibly a problem with the subs. What ohm load are you running on what amp? >This info could help get you some more info. Seen that happen before and system eventually quit for good! I would find problem soon!