I don't know if it's related to RabidVWM. Upon switching to my user and programs returning from being paged whilst my hard drive is churning, I suspect that VWMSwitchOnFocus sometimes switch desks. I'll make a note for next time this happens to be more observant of what's going on while my screen tears.

EDIT: I just wrote one. It's going in front of the crumb-wreathed monitor base. Let's see how long it stays there.