Bourne fails to make child support payments, so the IRS and the FBI have teamed up in a joint venture called Briarstone to bring Bourne in.
no, but seriously. The reason why Bourne went to Treadstone to forget his past wasn't covered at all in the movie. He was living in Cambodia with his wife and 2 kids when his village was bombed. He was out taking care of something and when he came back, he found his wife and children were killed. No governments wanted to take responsibility because Cambodia was neutral in the war, so he couldn't get revenge.