Probably drivers from the local transit union. I've seen/read about (union) activists doing some crazy things and it wouldn't surprise me that someone or some people would try to show with their life that this is a bad idea and therefore shouldn't replace a human driven system.
Do you have any data to back the assertion that this was about unions?
My gut says that it’s more probable that it was the hacker news crowd trying to test the technology, but I have to admit to have absolutely no data to support this assertion, either.
Was it on the streets, with no traffic blocking ? I took the same kind of shuttles as part of an experiment, but it was on a very specific path that was shared with pedestrians, not on the road.
They ran a test in Gothenburg as well at one of the university campuses. And while some of the roads they ran on where technically public car roads, they where small back roads across campus that see very little car traffic.
"We where very surprised to see how many people intentionally jumped in front of the moving buses to see if they would stop"