Could just be the plants hardening off from the additional light, could also be over watering? how long til you flip? my personal preference the week before I flip I citing off everything below the top 3 sets of leaves, excuse the photo post but it's for an example of how much they explode once I do that and flip.
that is the first week of 12/12, it's scarey to the new grower to remove so much growth but it is completely fine and you will be very happy with the results! Gotta cut put that undergrowth that will only sap the energy from your main colas! your grow is looking great man. nice first run