Usually it's picked on something or it lost handle which may cause the rock to maybe start turning in the opposite direction. Or they just misread the call and threw the wrong turn.
I'm referring to when the rock simply curls the wrong way. Consistently, no matter who is throwing it.
It seems to be related to uneven cooling of the ice. It really becomes apparent on the southern-most sheet in the club I frequent when the late-winter sun starts beating down on the southern-facing wall next to it, but that is not the only place I have seen it happen.
However, outside of the high level reasons for it happening, I am interested in the specific mechanics, like were previously described for normal curl, that causes the rock to go the opposite way under these conditions.
As I mentioned in a different comment, it's probably just gravity. The rock slides against the curl because the ice is not level. Uneven cooling could cause the ice to freeze unevenly, or maybe even sublimate. Shaving patterns over time can also play a part.
Of course I'm speculating on the specific underlying cause, but I would be pretty surprised if the mechanic isn't just the rock sliding downhill.
