i was reading BOFH and he told someone to rapidly switch their power supply on and off, and it made it explode.
why does this happen?
just curious.

Sproingie, you left out that the solution is to it is to reverse the shield polarity.

It happens because it's fiction and the writer could write anything to make it more funny.

A powersupply contains a capacitor and capacitors may 'explode' under certain circumstances.
Google for 'capacitor explodes' if you want to know more.   

Rapid changes on current flowing through components. Some PSU have some high farad capacitors inside. Really, even one explode can surprise you not mention 5-6 of htem in a small 50 celcius box.

So if i understand correctly most of you are unabombers (joke)

I had a GameBoy Advance XP power-supply, that suddenly went bang, really loud. After that, it started smelling and I turned it off. 
Security first.

It didn't explode though - it was intact to the eye, but non-functional.

Cheap power supplies tend to use capacitors that have a voltage rating only just above the working voltage and therefore are more liable to go pop.  This is particularly likely to happen if you switch on the power supply before connecting it to the equipment it is powering.  This is because the output regulation is often poor in the absence of a load.  This over-voltages any cheap capacitors.

Also electrolytic capacitors rely on the aluminum oxide layer for insulation.  This tends to degrade if the power supply isn't used regularly and reforms during use.  Thus a cheap power supply that hasn't been used for a few years may well go pop when you switch it on.

If you have an old piece of kit like an antique radio you want to get going, it may be worth using a variac to reduce the applied mains voltage.  This applies less voltage stress to the capacitors and they are more likely to reform rather than going pop.  After a few hours the voltage can be raised back to normal.  This tip won't work with modern switched mode power supplies, because at low input voltage they'll either fail to start, or (or 110-250V input types) they may operate as normal. Generally I wouldn't try this on computer kit.  Any experimentation is at your own risk.

Alan

@Mads, I was playing Tetris for the GameBoy.  I remember when the batteries were getting weak the game would get glitchy.  I'd turn it off and then back on and I'd be good to go for a few more minutes.  After doing that a couple of times the batteries exploded.  I had battery juices everywhere.