I have had that happen on 2 seperate occasions... let me know if either of these applies to you (a) accidentally/intentionally used soil with fert in it- whether it's time released nutes or miragle grow soil doesn't matter (b) there are multiple bulbs either too far away or too far apart which I learned causes the leaves to freak out and grow in both directions to absorb from all light sources. Like I said, if either of these applies let me know, and I'll walk you through fixing the problem