Cassius is angry at Brutus for scolding him and treating him like an inferior. Cassius doesn't appreciate how Brutus will try to give orders to Cassius's army and makes suggestions that Cassius's leadership on his army is suspect.
Brutus is upset that Cassius's army has been looting and takes bribes from villagers as they march along. Brutus feels that he's a noble enough person to run their own armies without resorting to viles means for monetary support. Ironically, this isn't the only thing Brutus is mad about. Brutus is also upset that Cassius didn't send money when Brutus requested it. Brutus's army is running out of funds, and he needs Cassius to help pay for the cost of his own army. Brutus apparently chooses to ignore that the money he is requesting is that same money that he doesn't believe Cassius should have in the first place.
Who's to blame? I don't know; it's a pretty childish argument in the first place. (They even resort to name calling - good stuff). I think the Cassius probably has a more legit reason to be mad simply because of the contradiction of Brutus's anger. Brutus would have a good reason to be mad if he wasn't also upset about Cassius not sending money. Either way, it really doesn't matter who is more right or more wrong; the important factor is that these guys should be bonding in order to win a big battle instead of fighting like third graders.