I really don't know. Does it give a message when creatures who're not part of your fortress throw a tantrum?
Because I didn't see one. I also don't know at what point they actually start throwing a tantrum.
Of course there was also a lot of stuff lying on the bridge, but I doubt that anything being thrown around on/at the bridge would cause it to deconstruct?!
I'm currently "testing" this again with a goblin bowman being trapped down in the same spot.
He is currently at 96046 stress, rising more slowly though. Hopefully the newest siege remains will help him get there faster, but I guess he might already be immune to that.
I could also try some plump helmet men or gorlaks, but I prefer to not cause them harm, it kinda makes me feel bad. Troglodytes at least fight back, but I haven't had any of them in the caverns lately.
So far I can only explain the sudden destruction of the bridge with one of the troglodytes throwing a tantrum...
EDIT:
I'd say the platinum bridge is more deadly for creatures impacting it than the gold bridge was.
EDIT2:
Sadly the goblin bowman down there only went haggard, stumbling around obliviously, when he reached 250k stress.
(Also means that, yes, 250k stress is the point when they break.)
But this really makes me think that the troglodytes destroyed the bridge under them when they finally snapped and started throwing tantrums.
Quite concerning, when you take bridges for being those indestructable seals from inconceivable amounts of !!FUN!!.
Better keep those dwarves happy - or at least away from bridges!
EDIT3:
Hm...my dear test subject poor goblin prisoner is now at 518k stress and currently throwing a tantrum (after he kept "stumbling obliviously" a few times and "sunk into depression" at least once). So far the bridge is still in place, luckily. I wonder how high stress can go, since ... well, there's no way he'll kill himself, unless he manages to demolish that bridge under his feet or one of the two iron bridges blocking entrance to his "prison".
EDIT4:
Tantrum over. Bridge still intact.
Then again, he has ~5000 items to play with around him...it is still the garbage dump where he's imprisoned, maybe he just didn't get to "target" the bridge?
EDIT5:
The bridge just disintegrated after some time now. Didn't even have magma below it anymore. The goblin wasn't throwing a tantrum at the moment of disintegration as far as I could see, but it definatly has to do with him being on it, because it never happened while no stressed creature was on the brigde...
Well, stressed goblin still alive, now to figure out how to rebuild the bridge with him being down below. Sucks to lose the 3 platinum though, but heh, guess when my experiment is over I'll just have to pump out the magma again and get the platinum bars from the bridge.
Really don't understand how a stressed creature can just destroy a bridge it is standing on.