Why was Auggie mad at Jack?

Without knowing an exact chapter, it is difficult to answer your question precisely. One example of Auggie being upset with Jack can be found in reference to their Halloween costumes, and a comment made to other students by Jack about Auggie.

1 New Text Message From: AUGUST Dec 31 4:47PM
got ur message u know why im mad at u now?? did Summer tell u?
1 New Text Message
From: JACKWILL Dec 31 4:49PM
She told me bleeding scream as hint but didn’t get it at first then I remember seeing bleeding scream in homeroom on Hallween. didn’t know it was you thought u were coming as Boba Fett.