OK fine.
Here's what I assume happened then: the gnomes are clearly out to catch your bear. But they know that the bear isn't likely to just walk up and let them grab it. They know they can steal the bear from your carboy, but we've been on to that for some time (see DrunkleJohn's post). What not everyone knows, however, is that bears are strongly attracted to the scent of carmelized malt, particularly on the top of a hot stove. For this reason, the gnomes will often turn up the temp on your BK when you're not looking. The kettle boils over, the bear comes sniffing around, and the gnome grabs it.
As confirmation, let me ask you this: on those occasions when you do have a boil over, how many times have you had trouble finding a bear for your fermenter afterwards? If you're anything like me, it's very difficult to find a bear on those days, and I'm pretty sure the reason is that the gnomes have already stolen the bear, by luring it in with the admittedly heavenly scent of burning wort.
Case closed.