"Bunch," when used for a bunch of people, might be considered a collective noun. We might say, "A bunch of people are coming over tonight."
However, when "bunch" refers to things, like a bunch of bananas or a bunch of grapes, the verb is almost always singular. In the New York Times Archives, I found many examples of "a bunch of" + a plural count noun + a singular verb (a bunch of bananas / flowers / grapes was), but only one with "were." Here it is:
So, I think in this case, the word "bunch" can be considered a collective noun used with something inanimate. Gilbert, you have come in first with a correct answer to this puzzle.