I had the same issue with my plants. It happens when one of two things happen, sometime when the lights first come on in the morning from the night time they are not reaching towards the light so the leafs appear to be just hanging down then after about an hour they are reaching for the light again. Also this happens when they need water, sometimes I dont come home on the day that they need to be watered and they are thirsty and they are drooping down. For the people that have their plants falling over you should tie they up with a stick so they have some extra support and put a fan blowing on them to promote strong stem growth.