The reality was, Brutus stabbed Caesar and apparently was part a plan to do it. That was treason and the ultimate in personal back stabbing, "Et tu Brute!"
Brutus and Cassius were at war with Caesar from time to time before that. The biggest fallout that Shakespeare mentioned in his play was the conflict between Caesar and Pompeius, the latter of whom Brutus and Cassius sided with. Some sources claimed that Brutus was pardoned by Caesar because Caesar suspected him to be his illegitimate son.