Could be way too many things to list, but more than likely it was a power surge either through the bus (unlikely) or through the telephone line.

Most decent quality surge supressors (which is different from a power strip) have a port for incoming and outgoing telephone connections. It will block surges/spikes on the telephone line.

Another possibility is that it was just its time. A modem is chip circuitry like all the other components and have the same problems with heat and age as the other components in a system. But unlike a hard drive or printer there are no moving parts, so it should last a very long time prior to giving you problems. I would look at the first possibility and check on your surge supressor.

But remember.... "sometimes people just explode..."