A blown speaker may be the result of a mechanical failure, such as when a woofer comes loose, or a thermal failure, due to overheating voice coils. Typically, speakers get blown when they are pushed to their limits in terms of volume.
A mechanical failure can occur if the cone of a speaker is being extended further than its designs were set to do. Repeated movement during this can cause speakers to become stressed, and the vibration can cause damage. The most common part in a speaker that causes mechanical failure is the woofer. Thermal failure usually occurs when a speaker is supplied with more power than it is designed to hold, causing wires and coils inside the speaker to melt or burn.