Actually, this has happened once before now that I think about it. I had a boil-over like this with 5 gallons of wort in a 30-gallon keggle, at a group brew day in early 2009 when the guy whose house it was let me borrow his gear after he was done brewing. Hop pellets in, explosion of foam. Damned if it didn't get hop sludge all over his whole expensive rig and nice clean garage despite a spray bottle, fermcap, hop sock and 25 gallons of head space.
My best guess was I had the propane burner on too high last night. I have never boiled at full heat before, but I had collected too much wort and my efficiency was a couple points low so I wanted to boil off a little extra to try and hit my target numbers. Plus I had had the perfect brew day up until that point. Hit all my temps, no stuck sparge, even managed to nail the decoction. I was so confident, and so used to everything going smoothly once I got the boil going, that I just walked away and cranked up the Wii, setting a timer for the hop additions.
I managed to clog the burner so badly with hop sludge that the electronic spark won't start it anymore. Had to use a match to relight the burner last night. So I may have to go buy a replacement before the next brew day.
The silver lining to all of this is that I grilled the perfect strip steaks while I had the IC going.
So the lessons here are 1. Don't boil at full heat, 2. Watch the freaking kettle, 3. Get a bigger pot, 4. Keep the fermcap refrigerated, 5. Use a hop sack.