My guess is…chip thinning at play.
At 72ipm with a 2 flute at 12.000RPM, chip thickness would be 0.003" if you had 50% WOC or more (>0.125"), but at 0.04", chip thickness is much lower due to chip thinning.
Depending on how sharp your cutter was, you may have ended up with an actual chip thickness small enough that the cutter was half-cutting half-rubbing, which made it bounce on the wall, producing chatter. Increasing feed rate to 100+ ipm effectively increased chip thickness (that’s exactly what chip thinning compensation does), enough that the endmill started taking large enough bites that it plowed through material and stopped rubbing/bouncing on the surface.
The X versus Y difference may just be related to wood grain ?