While SSD is right we don't have an AoA indicator in most light aircraft.
The mechanics of the killer spin off the final turn are quite simple.
Assume a LH circuit. Pilot is on base, approaching the extended centreline. He rolls left into the turn and realises he's going to overshoot the centreline so he tightens the turn a bit (maybe he misjudged it, maybe he has a slight tailwind component). So he's now in a steep turn. At 60 deg bank the stall speed (alright the speed at which we reach critical AoA) is getting on for 1.5 times what it is in level flight (if you want the maths behind that it's
here)
Because of the turn the left wing is going slower than the right wing so that will stall first. The result is a sudden roll to the left and a near vertical arrival.
As an example an Arrow III has a stall speed in the landing config of around 55kt. Heel it into a 60 deg bank and it will become 77.8kt.
Food for thought.
Mike
edited for incorrect numbers