Biff was more than angry about the incident in Boston. He was furious. It was enough to keep him away from home for many years, and it was enough to make him throw away a good future him and his father had been discussing throughout his highschool years. It was enough to throw his "life" away. Catching your father cheating on the mother you love and respect more than any other woman in the world is a harsh situation to deal with for any child, and Biff has already had a rocky childhood. Also, Biff thought of his father as a hero. As a truly magnificent man, and finding out that he was cheating on his wife opened Biff's eyes to an uglier image. A stark image. One that I'm sure he did not want to see. After this incident, he sees his father as a phony, a fake. He is not a magnificent man, no, he can't be, because he's a cheater and a liar. It was a rude awakening for a man of only about 17 or 18 years old.
Biff is no perfect man himself though. He has a little issue with stealing things that are not his own. When he was younger, it was small, insignificant things like a football or some wood. Biff was never taught that stealing was wrong, Willy had convinced him that these small things were perfectly alright, that he could get away with it, not thinking of the possible future repercussions. This led to Biff getting into trouble when he was an adult for theft, even serving time in prison for a couple of months. It can be said that the incident in Boston is what sent Biff on his downward spiral, and Willy believes that Biff is blaming him for his wayward actions that he disapproves of. Going off and working on a farm is not what Willy wanted for his son, but Biff took off after the incident in Boston. It is uncertain of whether or not Biff truly blames Willy for the way he ran off and how his life has turned out, but it can be assumed that it was a factor. This is because Biff had had plans to make up his grade in math before his trip to Boston, and when he came home, he no longer had any. It is understandable, under the traumatic circumstances.