Assuming there is nothing wrong with your sail or mast, this can happen when you tighten the outhaul before the downhaul. If you set the outhaul to about the right tension, especially in heavy wind, then tighten the downhaul, the bottom of the sail will pop out. The geometry causes the tension on the foot of the sail to increase as you increase downhaul pressure. To avoid this problem you should downhaul first and then bring in the outhaul. In heavy air you won't be able to get the downhaul in tight enough from the tramp on the trip downwind, so you will have to leave a little extra slack in the outhaul knowing that the final application of dowhnaul will pull the foot tight.

Eric