“Bun fight” is pronounced strikingly similar to “bon fete”, a well-wishing phrase much in evidence in Ottawa on our bilingual Canada Day yesterday. Could it be one of those loose borrowings along the lines of “loo” ie “gardez l’eau”?
This colloquial thread might give further credence to the term as a Canadianism:
http://discuss.hancockwildlifechannel.org/viewtopic.php?f=6&t=1315&start=3200

Bun fight means a chaotic situation, as in it was a complete bun fight. It originates, I think, not from buns which are a sweet tea time confection but from bread rolls which were thrown either at a university dinner or perhaps at a private school like Eaton or Harrow. I think it was a regular past time at the Drones Club, particularly on Boat Race night according to Bertie Wooster.