Fort suddently died, looking for an explanation

I had reclaimed a fortress in a hot region, and started to dig looking for water. Then a message about the fort's death appeared. One tick, I had 7 dwarves happy digging, farming, brewing, in multiple layers of cavern, and one tick later (a looong tick, the game froze a bit) and I got the fortress death message.

I did not have a savegame since not even a season had passed.

I don't have any mods. Just ironhand tileset. After reclaiming again I've got a hidden collapse, skeleton of dwarves and a dog on upper layer, and just some bag in the lowest layer where miner was working. No cave or other !!FUN!! has been located.

I'm puzzled.

EDIT: Another collapse on level lower than I had dug. I'm puzzled and scared.

EDIT: Corpses are found "dehydrated", but dwarf was fine and a lot of booze was there.

EDIT: Dug to the collapse, seem unrelated as I found a seasonal flooding magma sea and water. Also a lot of nether-cap inside the magma sea suggest that the first winter's magma flooding has filled the cave causing the collapse somehow, maybe the first time meeting the water lake.