No need for the wing to move down. Just moving the aileron down can
take the wing tip AOA past Cl-max into stall, especially if there is an
upward flow component at the tip relative to the rest of the wing.

BTW, it's not just the tip, you can stall the whole wing too. The SGS
2-33 is prone to do this when entering a big strong core. I suspect it
happens more often in big slow gliders because the same updraft causes
a larger change in AOA due to the slow speed, and the inertia slows the
pich down from longitudinal stability.