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.

If you make a copy of your save, abandon your reclaim attempt, and export historical data with Legends mode, you should be able to look up what exactly happened to your fortress. A legends viewer tool may help make this easier.

From the sound of it, there was a cave floor collapse, possibly caused by digging in the wrong spot. This usually kills the miner digging and anyone below the collapsing floor, and sometimes the cloud of dust caused by the collapse suffocates everyone nearby.

Dwarves can die from the heat:

In extremely hot regions (above 110°F), dwarves and livestock may die from the heat (specifically, the fat in their bodies will melt, causing them to bleed to death) if they remain outdoors for extended periods of time.

Remember that your embark square could have multiple biomes - it could have shown as "hot" but also contain a "scorching" biome. To be sure, press F1, F2, etc. to cycle through the different biomes before you embark.