Just a guess, but the dead bees might have been formally clustering over the brood and couldn’t get to enough honey stores as your temperatures got back to normal. As they ran out of energy (trying to heat the brood to 95F) they dropped from the comb and died from the cold. The energy expenditure of the bees is probably at a maximum in the spring because they are trying to keep the brood at 95F while the night time temps could be dipping in the 20F (25F here in Michigan last night). If they are too cold or too stubborn to move to new stores under these circumstances, they are going to expire.
I’m guessing in MA, you probably had some insulation of some sort over your hives in the winter? My advice would be to keep that insulation on until spring really sets in (Late April, May). No, it won’t hurt to keep the insulation on in the spring; I keep 2” of insulation around my hives year round. In the dead of summer, my bees still have lots of foam insulation around them.