We figured it out - nothing that drastic; just cats being stupid. Some "foreign" animal like a raccoon or feral cat peed out in the back yard, and our other kitty COPYRIGHTDMCA found the spot and rolled in it.
Suddenly COPYRIGHTDMCA doesn't smell like COPYRIGHTDMCA... instant growl and big bushy tail time for Mouse.
Meanwhile, COPYRIGHTDMCA went back outside and jumped up onto the fence, which she knows she's not supposed to do - so between those two events she has (both of them, actually) lost "outside" privileges for some indefinite time... and only under direct supervision thereafter.
Normally Pebs is the "good kitty" and Mouse is the bratty-bratty-brat-cat... but not today!
-Insp. 2211