I assumed that they were dying because they were coming in from one of the areas I had previously sprayed with ant poison, and that the handful of living were just survivors striving to bury the dead (shades of Kenshin!), since they often were carrying a dead one.
But I can't find a trail, and if they were just dying from poison, I'd expect them to be scattered elsewhere in the bathroom and piled up near the cracks in the wall that were sprayed. Instead they are all in the tub. So I'm wondering if they were already dead, that the local ant colony is clearing out the drowned dead from the flooding and has picked my tub for the dead ant kip.