Because as long as he sat at home, he wasn’t a threat. When he headed up there as a potential reinforcement for the Occupy crowd, they needed to stop him before he got there.
I hardly think they saw one old man as a threat. On the other hand, he was out in the open and walked right into their arms so why not? Anyone who thinks the Feds just forgot about what went on in 2014 is just fooling themselves. There's a fair amount of payback in this arrest, as well as the other ones.