Common term for when a gathering of Bro's goes Awry.
Often, this term is invoked at a bar or in a social setting in which a group of meatheads gets out of control or causes displeasure to those around them. Also, it can be used to describe a lame social setting such as a bar overrun with douche bags or a more urbane situation that has been ruined or made less enjoyable by a group of bro's.
If you are part of a group of Bro's it is acceptable to use the word as a sort of rally cry or an ironic pronouncement of a situation you helped make less enjoyable.
This was a fun party until that group of douche bags rolled in and made it a complete Brotastrophy.
Bros, i can't wait until we get to that party. We're gonna grab so may tits it'll be a Brotastrophy.