The pathway they were on, plus the traffic on the street. They were asked to move, refused, then when told to they failed to obey the lawful direction of a law enforcement officer. At that point the officer has the right to detain the individual, even using the tactics used.
The bottom line is play stupid games, win stupid prizes. They were told to leave and didn't, therefore got what they deserved. Just because you disagree with the law doesn't give anyone the right to ignore it.
By the way, a similar protest here included two individuals legally carrying long arms. The local police also showed up and told them to vacate the overpass for the same reason. The individuals did and went on their merry way...