Depends on what they were doing. If there's nothing in the combat log, check your food/drink stocks and see if your dwarves have an unimpeded path to them. Failing that, you could have a vampire, but buggy pathfinding is the most likely culprit. You'd think the most common cause of death would be combat in Dwarf Fortress. But for me it's actually my dwarves climbing trees to try to attack a barn owl or something, then being unable to get back down without someone chopping down a tree, and finally dehydrating/starving to death. A close second is Dwarves chasing things into pools and drowning. The 3rd most likely cause of dwarven deaths are people using the architecture in ways for which they were never intended; like jumping down wells, or ignoring that low traffic path and getting run over by a minecart, or jumping/getting knocked off ledges by people/monsters/idiocy/etc.
If your dwarves are getting stuck in trees, cut down the trees periodically, it's not like the elves and their wooden armor have ever been a serious threat to anyone. If your dwarves keep drowning you might want to create your own, artificial pool where they won't be interrupted by things and forget to to paddle (or whatever). If you have dwarves just getting killed by random creatures/monsters/etc. then limiting the time they spend outside with burrows and other such things is a good idea. If you have a vampire look for the dwarf with the thought "Urist McVampire was horrified after seeing Urist McVampireChow die" as that's a dead giveaway that they did it. Or if the vampire has been in your fort a long time, look for the dwarf thinking about not having alcohol in a long time. Vampires can't drink alcohol, but still get alcohol withdrawal; because of course they do.