Background:

I am using a Si7456CDP N-channel MOSFET in a switching power supply. The power supply and load are housed in a plastic enclosure. Yesterday, the power supply and load were working perfectly. This morning, when I came to power it on, nothing worked. No power. Eventually, I discovered that the MOSFET's source and drain had shorted together. Replacing the MOSFET fixed the problem.

Question:

What could cause an N-channel MOSFET to suddenly fail with a source-drain short?