branches falling off

[duajones]

of off several of my seedlings. Potted up a week ago using what turned out to be poor quality potting soil but plants looked ok afterwards. But last couple of days a few of the seedlings have dropped whole branches near the bottom of the plant. A couple of them I just barely touched and they fell off. Is this a damping problem or what? Plants are somewhat leggy but look normal otherwise. temp is around 75 degrees and i have a small fan that I rotate between the two sets of plants. Lights are within an inch or so of the plants as well.