Same as USA 197 says but I know this problem in combination with mainsheet pressure.

Comes from the boom having an angle relative to the foot of the mainsail. It the foot is tight when the boom is still angled upwards somewhat then applying mainsheet tension will straighten the boom (relative to the foot) and thus require the foot of the sail to stretch (which it can't) or pop out of the sail track (which happens instead).

Solution : know where to trim the outhaul before indeed applying mainsheet (downhaul) pressure.

Wouter