1. They arrested the wrong guy from the 7:15 shooting and the second shooting was by the same lone gunman.
or
2. After the first shooting was over, the Police were sweeping the campus for evidence, etc., came across the second guy, who got paranoid, thought the police were coming for him and started shooting.
Good guesses. What about
3. It’s a conspiracy
??
3rd possibly, two “whackos” that decided to pull off coordinated attack