Could be stem stretch.. If you have the light too far away, the seedling or plant will stretch to reach the light, and get too tall to support itself with such a thin stem. Or, if you have a light with the wrong spectrum, like an incandescent bulb, (too much red), it will also cause the plant to stretch out and be weak.
Maybe you could flower earlier, top or tie down your plants, SOG, SCROG, there's so many things you could do to control height. All the info is here too. Grow Journals will help...Thats whats helpin me on my first one.
I don't have any light on them except for natural light at this point. I don't have to many plants started at a time, but I have the same problem just about every time that I do this. And my friend who knows how to plant them has been having the same problem.
use a fan if inside, increase light from directly above. If it falls over than add a stake to it. Tie with string and over a couple of days allow the sting to be looser while adding more wind power. The heaviest of plants can support their own weight with enought wind while under veg. Plants should move back and forth under wind power but be able to stay up. If not reduce wind and increase as they are able to take on more.