Why not? Were there not a couple of hundred people on the field and sidelines who this man, a man with highly erratic behavior and unknown intent, could have potentially hurt?

What if he then ran towards the stands and jumped into a section full of fans -- again, without it being known what the erratic man's full intentions were?

It's perfectly reasonable for any person (including Bobby Wagner) to feel this man, considering the man's behavior, posed a potential physical threat to others, and reasonable for a person to do something about it.