It could be a phosphorus deficiency or other deficiency of more then one nutrient, or it can even be a result of under watering maybe, Cant be exactly sure though. :wall:
I would try either lowing temp and humidity a bit and see if that helps, or watering them only once every other day and see if that helps, if neither help at all after at least 3 days then you may need more light or more nutes if you think you haven't been giving them enough.
I of course could...