Why did Benny suddenly get mad at Pink about not signing the pledge at the end?

Benny was the only one who really gave a $hit about the team.

Benny was just frontin in front of the group, but one on one, Benny laid down the real to Pink, because Benny was the only one thinking about his future. The other players may have seemed to support Pink, but in reality, Pink was the only one to not take the pledge.

Football in the American South- a true religion

I think despite Benny being shown as a dumb, drunk brute, he was the only one who figured out that Pink was serious when he said he was thinking about not playing football that year. The other guys either didn't take it seriously, or just thought Pink was rebelling against the pledge. None of them took the pledge serious, but signed it to keep the coaches off their backs.

I forget if it was a deleted scene or in the actual movie, but at one point Benny says about the stoners along the lines of "They don't care about the school, or the team." echoing the sentiments that the coach has about Pink hanging out with Slater, Pickford, and Wooderson.

It's his big year and this is a threat to how he sees everything going on for what is being built up as the best year of his life. Without much else to go on it's pretty likely that Benny is going to be one of the many guys in the 1970s that went from graduating high school straight to the local factory job where he's going to be a drone for the next 50 years of his life. He's all in to getting as much out of it as possible. Even Benny says that it's what "[Pink's] been working towards his entire life." While it might not be Pink's ambition it speaks volumes about Benny projecting that kind of thing onto him.