Bee swarms; can you explain this?

A bunch of years ago, I was on a blacktop at a park, when out of no where a huge swarm of bees (hundreds, at least) flew across the park and through the blacktop. I remembers seeing it from a distance and running out of the way to avoid getting hit, but there were a few people who were standing dead-center during the blast. They got pelted with a bunch of bees and ended up freaking out. They all moved together, flying around, and I'd never seen them landed. This happened a probably a few times in the early summer at the specific park, and no other time of the year. I've only ever seen it happen twice while I was there, but both times were pretty scary. Someone explain this, please. Is this natural? Why??