Yeah, there is probably a nest up there with an egg in it. He probably figured that you were planning to make an omelette from it. I have had birds fly by and give me warnings to stay away. They were so close I could feel the air pressure change from their wings, but they never touched me.Posted by Mike (here)
I have no idea why it attacked. Surely it didn't think it was going to fly off with my fat ass. There are a few nests in the tress bordering the parking lot here. Maybe that was it. But then again, I've walked in those areas numerous times without anything like this happening. What a weird thing to have happened.
Ospreys are endangered birds so it's probably good you didn't find that rock.