Dead-out in October with honey still in the box and heads down in the cells — starved ten centimetres from food?

Galway, second year. Lost one of my three. I went out on Wednesday, saw no activity at one entrance on a mild day when the other two were flying, and opened it up. What I found. A cluster about the size of a grapefruit on two frames in the middle of the top box, all dead, and a large number of bees head-first down in empty cells with their abdomens sticking out. Two frames over on the same box, sealed honey — maybe three or four kilos. No smell except wax and honey. No mould, no chewed comb, no mouse. From reading here I think this is textbook starvation: heads in cells is what bees do looking for the last drops, and the honey being present but not touching the cluster is the classic isolation problem. But it's the 21st of October. We've had nothing colder than about 3°C and they've had flying days most weeks. So what am I missing? Could a small colony starve in October with food two frames away in a mild climate, or should I be looking for another explanation — a failing queen leaving them too few bees, or mites? I'd rather learn the right lesson than the comfortable one.