Everytime when my plants do that it's due to heat stress from the light being to close so they sweat like humans do in order to cool down or if the buds are to close to each other and the moister can't evaporate.
yeah mate , that is what I thought for a while, but it just seemed to happen the same time I gave them a higher dose of nutes. but it could be! cus I had the 600w quite close like 8-9 inches (cooltube) I have since moved it to 12-13.