If you can (maybe with an earlier save), check the status of one of the dying / about to die units (v-z for pets, v-z-Enter for dwarves). What does it say? That could help narrow down the cause of death - if they've got rotten body parts according to that screen (which often don't show up on the Wounds screen), then it's probably a contaminant they've got on them. If they've got melting layers/parts, then it's probably a heat issue of one kind or another (as has been discussed above).
And it still could be that titan. While I haven't dealt with many titans in 31.x, they seem to largely use the same systems as the Forgotten Beasts, in which case the titan's blood, ichor, extract, or other goo could easily be the cause of issues. Utterly bloodless, goo-less FBs and titans can happen, but are very rare in my experience - usually they just have goo that you don't notice the effects of, or they're harmless in the first place. You mentioned it died by freezing in water, so there could've been ichor left along the titan's path through the water to get to that point, for example.
Way ahead of you. Earlier saves show nothing on the wounded screen, or the status screen, although one of the cats that dies later is missing a leg. Dunno how that one happened.
The titan thing really isn't possible. He fell into the brook on the other side of the map, and it froze 3 steps later. The only reason that the brook was channeled through there was for my own testing purposes. I've just come to the conclusion it has something to do with the magma I created in DF Liquids.