I attribute it to weather. I see a lot more molting parties during the warmer months. Especially considering you (more than likely) have them all on the same feeding schedule to boot, pretty normal for molting cycles to sync up.
It's moulting season. Although we probably keep them around the same temperature all year long, there could be other triggers... like amount of natural sunlight and barometric pressure. They are preparing to breed in the fall.
I have always experienced an increase starting in April and May, with a huge spike in moults in June, July and August, and a slowing down in September and October. November to March is just the spiderlings moulting for the most part. The majority of my adults will moult in June-August.
Last July I had more moults than days in the month.
I have noticed this as well. My A. Metallica, C. Versicolor, B. Hamorii P.Cancerides and P. Cambridgei all molted this past week. The were all over 200 hundred days since their last molt. I thought it was kinda strange that they were molting at the same time.
Even though they are in captivity they are still sensitive to changes in barometric pressure, moon phase, weather etc and all the things that govern their little lives in the wild
My Ts know when it rains, they get active and hungry when wild insects are most active etc