What happened between Montoya and Pizzonia?

I’ve been following this morning’s Belgian Grand Prix via the BBC’s lap-by-lap reports on their website. I think I understand everything that happened, except for the Montoya-Pizzonia incident. Could someone who was watching the TV feed explain what happened? The BBC reported it thus:

Lap 42: Out of the blue, from second place, Montoya and Pizzonia clip one another – Pizzonia loses one of his front wheels, and Montoya ends up in the pit wall.

I know there were reports of drivers changing tyres throughout the race: was this simply a coming-together in the rain and mist? Anyway, congratulations to Kimi, and (I guess) to Alonso, who has almost clinched the title.