I did a quick search, and am not sure if this was reported in a thread like this yet, but badgers seem to be very, very aggressive creatures (especially for their size and relative lack of threat).
When setting up some fortresses for testing purposes, I seem to get a lot of combat reports suddenly popping up, indicating badgers had flown into rage (x7) and were chasing random dwarves or yaks around the map. Forutnately, badgers are fairly weak, and a single kick cripples one, but after they wake up from falling unconscious, they just go back to chasing dwarves again.
I'm not sure what the dwarves were doing (besides milling around the surface while my first tunnel was only partly excavated), but the badgers seem like some sort of hyperagressive dwarf-seeking missile that will hunt dwarves down in spite of dwarves doing nothing to really provoke them besides stand near them.
(And giant badgers are apparently something that would terrify a war elephant.)
Apparently, PRONE_TO_RAGE makes them very prone to rage, even when set at 1, because they are almost constantly flying into rages.
Amusingly, in testing this, I set 16 badgers against a single elephant, and they basically managed to somehow dogpile and knock the elephant unconscious, and very, very, very gradually nibble it to death. All the while, they were constantly flying into and dropping out of rages... even though the target was an unconscious lump just sitting there.