Stone-fall, weapon and cage traps will be triggered by most hostile entities entering their tile, with the possible exception of thieves, flying creatures and other occasional nasty fun surprises. Any unconscious creature will trigger traps, including your own dwarves.
However this does not seem to be true.
I had started a new fort, Usually I would use wooden Cage traps, until I had my hands on some weapons, then I would mix them with Weapon traps. In this embark I had no trees. so I thought I would start my trapping off with some stone traps. Just in case.
So I got the basic traps near my entrance, then I started to install a few more in crucial zones -- heavily trafficked corridors. I noticed occationally a message would say "Urist McDwarf has been found dead". At first I thought this was a case of me having a vampire (And that the Vampire death message had changed -- to make it less obvious.)
Then I thought I might check on the combat logs -- turns out my dwarves were being killed by boulders. So I thought: Must be the stone traps -- I guess I will pull them out and replace them with Weapon Traps.
That was !!Fun!! instead of a simple death from a stone to the skull, now the dwarves were periodically being sprayed across my corridor by the Spinning Discs. The deaths were no doubt quicker, but more brutal -- and Horrified anyone who witnessed them. In true DF fashion this sparked a tantrum spiral that claimed the fortress.
I never had a dwarf caught in my cage traps, but there were only 6, (half of which were in the almost untrafficed caverns.)
So something has changed in the way traps decide who to kill. I don't think my dwarves were passing out periodically on top of my traps.
What is the new condition for traps to attack/activate on your own dwarves?