I heard on the news that English wasn't his first language, which does present the possibility that he didn't fully understand what the cops were yelling.
Though the fact that he'd been living there a few years beforehand might suggest that he'd know it fairly well by now.
Though the most likely scene in my mind given the evidence, is:
People were in panic. Several people bust into the train un-uniformed screaming something this guy doesn't quite catch / understand. He jumps to the conclusion they are quite possibly terrorists taking hostages.
He decides his best chance of survival is to bug out and so runs. Unfortunatly trips, then the cops unload into him to the horror of the people around the scene.
It was an honest mistake by all parties involved. Tension and panic were high, this poor fellow followed his instincts, as did the cops which contributed to a bad ending.
The fact that there were un-uniformed police does contribute some concern however. Something I'm sure will probably be made sure not to happen again...