The main causes of subwoofer failure are;
1) over driving your subwoofer
2) driving a clipped or distorted* signal continuously to your subwoofer
*Clipping or distortion is the audible sign of an unclean signal and this can originate from any piece of equipment in the signal chain. What this means is you may be only driving your amplifier at say half power yet your head unit/source may be driving at close to it’s maximum output which could be supplying a clipped or distorted signal to the amplifier thus creating the audible distortion you hear.
This can result in either a mechanical or electrical failure.
ELECTRICAL FAILURE
With electrical failures the result is generally a burnt voice coil causing the subwoofer to go open circuit, or in some cases short circuit to ground. It is important to match the “RMS” ratings of your subwoofer to your amplifier to avoid over driving.
MECHANICAL FAILURE
Mechanical failure is when the spider or butyl surround rips or tears, or sometimes the voice coil ‘jumps the gap’. This means the voice coil is driven so hard that it leaves the gap between the voice coil and pole piece that it travels in and results in failure.
To prevent over driving or over excursion of your subwoofer it is very important to follow the recommended enclosure volumes, port lengths and diameter to ensure the optimum performance, and to protect it from these types of failures.