A lot of comments here, some correct, many incorrect.

First, United's PR department could not have screwed this up any worse than they did. Horrible handling of the incident.

Second. KEY POINT. Dr Dao had already accepted the voluntary denied boarding offer and was deplaned. When told that he would not be able to take another flight until the next morning (his VDB included a hotel voucher), he freaked out and at that point committed trespassing by running back onto the aircraft.
Why United PR never made this information public is beyond me. See point one.

Third, there has been discussion on Dao's big payout. From what I've been able to gather, he didn't get much. Enough to cover medical bills but that's about it. Note that his high profile lawyer(s) melted into the ether shortly after coming forth to represent him (Corby & Demetrio, where art thou?) … they likely reviewed the details of the event and chose to disengage from the $hitshow.

Fourth, the ONLY reason Dao popped back up in the news cycle is that his nondisclosure agreement expired. He's obviously angling for more money; fortunately, this story died very quickly after the NDA expired.

Finally, did I mention that United's PR department sucks?