Smokey mate, wow what a forest of Haze!
I got you PM,... Let me confirm, is this dropping starting just before sundown? If so, that's a common response, though I've not found out what's behind it,.. mine do the same thing, but it tends to stop during bloom...
If not, next thing is a water issue... you heft the pots all the time I figure, the weight is a big clue about how well moistened the soil is,... If a dry out happened, soil can be bitchy to rehydrate, patchy dry spots can form....and those side slits may let water out before it can soak in,... They have a huge amount of transpiration surface area in that canopy of leaves, and the pots are solid roots by now I wager, so drying out can happen bloody fast even if it's cool-ish... Did I mention using a wetting agent before, yucca or something else? I use that weekly, just to keep the surface soil from crusting up from drying, that and some light surface raking to bust up that surface layer....
....Over watering seems less likely, but if they got really soaked and stayed that way for days, some rot is a distinct possibility,... nothing to do there about that really,...
All that said, you my friend are spanking this grow! Those big girls are very nice, well grown, and are going to bury you in buds! This transition stage and bloom is the tricky part, shifting nute needs, pots/roots are getting stuffed making for fast changes in things,... Whatever you have for a micronutrient source, keep on that along with humic-fulvics, remember most of the micro's are immobile, steady low level supply is what works best... h-f's will help chelate them and keep them available,... Boron is a major player here, so is Si, they are facilitators to all the other nutes transport and metabolism, so they can become bottlenecks if deficient. Actual defc. may not show in the leaves right away, but it will still slow down the whole train!