As I understand it, Crane County was under a tornado warning at the time. Does it not seem reasonable that deputies would try to move people off public roads (whether it be a road or a rest area) and get them to go to some place of safety, like any substantially constructed building? While the storm chasers may have been conducting a business, it was the deputy's job to try to protect human life. If there was a line up of other vehicles behind the van seen in the video, all the more reason to get them to move out of the area. As has already been written, without seeing the whole video, I am merely speculating, as are the rest of us here.
Just as a corollary question, what do you suppose the reaction would have been if the deputy had just driven past this group of people, waved, and 10 minutes later, they were all killed by a tornado? Think he'd be in any trouble then? I'd much rather defend an arrest that I thought was justified based on the law and circumstances than have the lives of people I might have saved on my conscience for the rest of my life.