TLit,

Ahhhh, you may have rolled over one of those marble-sized stones just right to cause the tire to grip it and roll sideways. I've done that a couple of times. Scary as heck and just comes out of nowhere. Imagine jogging along on a sidewalk and you don't see a rolling pin sitting on the sidewalk and you step on it. Shwing!! I think that's what the tire does.

The last time was a couple of weeks ago for me. Same sizes stones at a corner and took the corner and the tire rolled over the stone just right to make the tire roll left suddenly which jarred the handlebars but I was hanging on and caught it.

Sounds like you handled it pretty well. Keep those eyes peeled!