This happened to a large organized ride I was on, too.

Yup, it's an attack. Screws, push-pins, and carpet tacks don't get into the bike lane by themselves.

Thank you for reporting it, hopefully it will get cleaned up soon. (they can also be a hazard for pedestrians and cars and drivers who park or make turns across the scattered sharps; but folks who put out sharps don't think about that...)

Don't let them succeed in making you stop riding!