For me, nothing would trigger it - wasn't anything anyone said to me or anything that happened during the day, but after a time of crazy happiness or high anxiety this horrible weight would descend with alarming speed, and within an hour I'd be trying to end it all. After medication went somewhat disastrously (paranoia, blurring of reality etc) I ended up developing a "survival kit" which I can honestly say saved my life.
So while it might seem completely senseless and selfish to those looking on, mental illness twists everything and genuinely depressed people (not drama llamas) are in a personal hell that only they can see.
(Sorry mods, just thought I had a duty to weigh in on the matter )
Back on topic, not exactly a bad habit but people riding when they're in a bad mood really annoys me - they get frustrated and either quit at the wrong point or take out their mood on the horse. I generally see this with spoiled kids who are too obviously being pushed by their parents, or people who have forgotten why they liked riding in the first place and have developed an obsession about winning ribbons at all costs.