It's because he was dressed like a Stormtrooper.

This isn't the hobby you're looking for. (via 7 News)

There's nothing wrong with buying an accurate Star Wars Stormtrooper costume and wearing it around town. You might get some stares, but that's probably what you want. There is something wrong with wearing it and loitering in front of an elementary school. That's liable to get you arrested.

That's what happened to this unlucky member of the Imperial Army. He appeared in front of Brickett Elementary School in Lynn, Massachusetts on Wednesday just moments before class was meant to let out. Parents there to pick up their children were shocked and afraid. They didn't know what he was going to do – he was holding a blaster, and they didn't want anyone to wind up like Uncle Owen and Aunt Beru. The principal noticed him and delayed dismissal until police showed up. Officers immediately arrested him and removed his mask, identifying him as a local man, 40-year-old George Cross.

Sources: 7 News Boston