Thanks all!

I should mention that this is an infrequent problem, but it happens just the same. Either wet conditions, or steep bumpy climbs where I stand up mid way to hammer it out. I will try switching skewers for these last few power rides of the year and see if the problem recurs on the particular bumpy, chip sealed climbs where this happens.

I hadn't thought to replace the skewer, because this has happened with at least three of the new model "external cam" design ones, and with two different rear wheels. The skewer is against the drop, so it's not an issue of long axel ends or a misshapen drop.

Time to go down to the lab and see if I can find a nice old internal cam skewer.