I think that if they are started indoors under a long photoperiod (e.g. 18 hours), and are then put outdoors whilst the photoperiod is still short (e.g. 14 hours) but getting longer then the changes in day length will mess them up quite a bit. I have experienced this quite a few times and just got very poor bud. I think that Maffro has posted about some similar looking problems with early plants last season sometime.
I suspect that the solution is to either start them in natural light (i.e. natural day length), if it is warm enough, or start them indoors under a photoperiod which is appropriate to the natural day length when you plan to put them outdoors (this is probably closer to 14 hours than 18 in April, but could be less). I plan to follow this strategy this season.