From yahoo Answer
What causes sub to blow
Subs tend to blow for one of two main reasons that I have found:
1) Voicecoil meltdown
2) Excessive cone movement(pop the cone)
If you have a very strong bass, which is wound up very loud, and assuming that the amplifier is NOT clipping, the speaker can blow simply because the cone/suspension/voicecoil fracture or otherwise separate. Suspension failure tends to me the most common - the cone tries to exit from the speaker due to excessive movement, and the suspension fractures around the extreme edge of the speaker itself - most are foam-rubber these days, but very old subs could still use corrugations.
This allows the speaker cone to move forward excessively, carrying with it, the voicecoil and former. These essentially then "Pop" out of the magnet, destroying the voicecoil connections in the process. This is what you MIGHT have heard as "Popping" a speaker.
It is a catastrophic failure, and the speaker is then no more then a paperweight.
Voicecoil meltdown occurs normally due to amplifier fault(s).
A classic example would be driving a good quality high-power sub speaker from a low power sub amp.
Say you had a 500W RMS sub speaker, and a 100W RMS sub amp. You can actually do more damage to the speaker winding the wick up on the 100W amp, then you can with an amp which is rated at twice the power of the speaker or more.
This is not to say that a 1000W amp won't blow the 500W speaker, cos it will easily, but the 100W amp driven very hard into distortion essentially deposits the amp's internal supply voltage across the speaker voicecoil.
So in those short bursts of distorted sound, when the distortion is actually audible, the speaker voicecoil is actually seeing bursts of the supply voltage at reasonably high current.
This heats up the voicecoil to a very high temperature very quickly, and eventually the coil gets so hot that the copper wire actually melts and breaks the circuit - fried speaker.
