The limited frame he showed had no sign of honey or nectar, so starvation is very possible. I have had hives almost starve in the middle of a drought. When they are in full brood rearing mode, it doesn't take much a food shortage for them to use up what they have and starve. I think some folks in the PNW will chime in about their issues last year.
Usually, in the starvation mode, most of those brood cells would have had the caps chewed open and and pupae still in the white would have been consumed, at least in the later stages. There would hove been a lot of wax debre mixed in with the dead bees on the bottom board as well as bunked bees in the cells.
Looks to me like it was a combination of starvation and toxic source that did the colonies in, 3 found the source and 2 didn't. The contaminated source might not have been attractive under normal conditions but were when the bees were in the early to mid stages of starvation.
I think it is a dual kill, starvation and possible persticide contaminated forage. It makes the most sense with the visual evidence presented.
Brian, so it's possible that with 2 hives side by side, one will find a nectar source and the other one won't?
My perception is starvation would be a gradual event, like running out of stores, then consuming brood as you said, then finally lights out. Could I actually inspect, see full frames of honey in the brood box, and then come back 2 weeks later and find a dead colony because they ran out of food?