Easy. Pedal bites into the pavement, forward motion carries it on through the downstroke, lifting the rear wheel off the ground. Presto, there goes all pertinent traction on your rear wheel.

Came pretty close to doing that once when I was green with roadbikes, and had a set of similar wide pedals on the '84 Raleigh Competition. Went through a left-hand corner hard and fast, with the left pedal down. Forgot to bring it up. Scraped like hell, and for a half-second, the rear wheel lifted. I was already putting pressure on the right pedal (split-second reaction to the left pedal's position - not a reaction to the pedal strike), hence, the left pedal lifted quick enough that the rear wheel came back down at an angle still shallow enough to recover from. Made it through the turn without incident.

That, my friends, is why I no longer use anything wider than a Campag Record road pedal (and on everything else, use MKS Sylvan track pedals). It was also the last time that Competition went out with those ungainly rat traps.

-Kurt