Hey, could be for a couple of reasons, what lights are you using? if its a floro then keep it 1 - 2 inches from the top of the plant, if its a HID of some sort then anywhere from 18 inches to 24 inches away. Secondly do you have a fan on the seedlings, if not i suggest purchasing an osciliating fan and stick it on them 24/7 that will strengthen the stems up and sort out your problem. Hope this help, good luck.

Happy Growing!