What could have caused my router to die?

The router was 5 years old and was a TalkTalk issue Siemens Gigaset. My PC was off and I was listening to an album on my media player's HDD (ie I wasn't streaming so the router was effectively not in use). All of a sudden there was a short 'parp' noise from the loudspeakers and the router's lights went out.

Luckily, I had a spare. Can anyone suggest what could have caused the old router to blow?

If its anything like a lot of modern electronics it would be built in redundancy. Designed to last a few years so you have to buy another one.

I've also seen routers killed through a surge down the phone line - often from a lightning strike - sometimes just kills the modem part, other times blows the router completely.