Initial thoughts from yesterday are that he had an engine failure at altitude that lead to a drop in hydraulic pressure. As the lack of engine would mean his airspeed slowing, he did not want to create extra drag by dropping gear. When he does get near enough to drop gear, no hydraulic pressure means he has to gravity drop them. Left gear eventually locks, but right does not and then the rest you've seen in the video.
Some of the press have taken it a bit far with lines of 'avoiding thousands of spectators' (I don't think it was ever that close), but to get it down and avoid a fireball the pilot did well.