As much as I hate to say it, I think it's the SPD pedal and shoes!
Before the SPDs or other pedals and shoes with recessed cleats, if you wanted to be firmly attached to the pedals, you either had to use cycling shoes with slotted shoeplates or the Look clipless pedals (which used large cleats that protrude from the soles).
The problem was you couldn't walk normally in these shoes, OK for racing and training but not practical for other types of cycling.