Either a) you weren't that valued an employee anyway or b) management is a bag of dicks. Maybe both.
Or he's going to work for a competitor. I put in two weeks last year to go to a job in a different city and state that would never conflict with the job I was leaving. I finished my two weeks, they paid my time and my leftover vaca and we parted ways.
About a month after I left another guy in the department put in his two weeks and was going to work for a competing brand, same city. They told him not to finish his two weeks and he got paid his vacation time when he left.