I'll ask maybe a stupid question: skewer properly tightened down?

As a poster asked above also make sure the wheel is "squared" in the dropouts, meaning both sides of the axle seated in the same spot on each dropout.

Not uncommon for a qr axle to move in horizontal dropouts a little bit under heavy (standing) effort or hard braking. Espcially if the qr isn't tight or seated evenly side to side.

Good luck!