I respectfully disagree with San Rensho. I suspect what happened was a simple bounce, not a slip.
Kooker - you already know about the "weight on your outside pedal" rule. At the same time you're pushing down on that outside pedal, bring your weight slightly off the saddle. If it's really bumpy/sketchy pavement you might even lift and hover completely off the saddle, but just barely.
What happens is, if your weight is fully on the saddle, you hit a bump with the front, no prob your elbows absorb it. Then the rear hits, catapulting your butt up in the air and un-weighting the rear tire, which then jumps off the lip of the bump and goes sideways.
If you instead float above the saddle, your outside leg acts as a shock absorber, allowing the rear tire to maintain contact.