Same thing that makes a glass squeal when you dunk your finger in water and run it around the rim - friction "make and break" - or, more simply, "slip and stick."
Check your rotors for glazing. If present, take them to a shop and have them turn about a thousandth off of the surface to remove it. While they're doing that, put a sheet of 120-grit on a block of wood or metal (to back it up and make it flat) and sand the surfaces of your pads to remove any glazing building up there. You don't need to remove a lot - just change the surface texture.
I just ran into this sort of thing with one of my wife's co-workers - it was caused by her being too gentle with her brakes - they'd heat up from use, but wouldn't wear down at all. I gave her the same advice - took me about five minutes of driving and talking to her to sort out the problem.
Fortunately, we caught it early - she may have been able to solve the problem entirely through a slight shift in driving habit (don't stomp on the brakes unless you have to, but be a bit more positive when you do step on the pedal.)
"recon" (sic - reckon)(tm) "hihgly"(tm) "seceed"(tm)
"Outback AIDS - Alcohol-Induced Dizzy Spells"