Because it work like only single subject.All staff work together like single person so here we use "was" not were.For example "the team is playing".
semd me mail if my answer is correct.
In British English, we often use the plural for collective nouns, though Americans often use the singular.
This is a hard one. I would use was just because "the staff" doesn't sound plural.
"was" is totally incorrect in this context. The clue is in the word "all"; this implies more than one so we apply the plural which means the answer has to be "were". Even if "staff" related to a single person then the sentence would still sound incorrect with "was". If the sentence read "The staff member WAS very angry" then this would be correct as we know that it is singular.
I wouldn't hesitate to use "were" here but I guess I'm not American.