TVtropes describes broken birds as "characters (often female) that are coping with a Cynicism Catalyst, a Despair Event Horizon, or a Dark and Troubled Past by becoming as cynical, stoic, and/or badass as possible". They are often the tsundere (belligerent yet soft) character, the psychotic and violent character, or the shy, introverted character. Most of these broken bird characters have gone through legitimate traumas, however, the easily broken bird's trigger was so thin, the reason for their trauma might as well be grapefruits.
They will fuck you up
In fiction, the easily broken bird tends to be limited to anime, manga, comic books, and animated kids shows. On top of that, they are mostly male. The trigger can be something as simple as getting rejected by a crush (as in the case of Sunako from The Wallflower and Hachiman from Oregairu), leading them to adopt unsociable personalities and isolate themselves, though the trigger often tends to be even simpler and stupider with the more extreme examples. Some examples of the more extreme examples are the Cluemaster from The Batman, who vowed revenge against a game show host because someone else had the nerve to win, Kenji Setou from Katawa Shoujo, who became a shut-in, anti-feminist conspiracy nut after falling asleep on top of his girlfriend while they were having sex and blaming her for it,Fukubei from 20th Century Boys, who forms a cult and kills billiions of people just because he wanted attention, and the All-Star Superman version of Lex Luthor, whose vendetta against Superman started because of a reckless endangerment charge.
Don't pull him over
So what exactly is it that would make people overreact to the simplest things? Adolescent illogic? Flanderization? Lazy Writing? Whatever the answer is, I'm betting $50 it's gummi bears.
Evil bastards!
Available sources:
http://tvtropes.org/pmwiki/pmwiki.php/Main/BrokenBird
http://en.wikipedia.org/wiki/The_Wallflower_(manga)
http://en.wikipedia.org/wiki/20th_Century_Boys