Sounding the horn in this circumstance was illegal since it was used not to warn the cyclist that he was potentially riding hazardously. Missouri State Statutes have the following to say about use of the horn:
Other equipment of motor vehicles--violations, penalty.
307.170. 1. Signaling devices: Every motor vehicle shall be equipped with a horn, directed forward, or whistle in good working order, capable of emitting a sound adequate in quantity and volume to give warning of the approach of such vehicle to other users of the highway and to pedestrians. Such signaling device shall be used for warning purposes only and shall not be used for making any unnecessary noise, and no other sound-producing signaling device shall be used at any time.
The SUV driver could have precipitated a crash by passing so closely while sounding his horn, rather than either leaving an adequate clearance or, if oncoming traffic was the issue, waiting for a safe gap before merging across the median.
Judging from the video, there was an approaching motor vehicle but some distance away so that the SUV driver could, in fact, have merged left in order to give more reasonable passing clearance. There doesn't appear to be any mitigating circumstance for the SUV driver's behavior, such as an oncoming vehicle suddenly entering the roadway from a driveway.
Contrast the SUV driver's behavior with that of the immediately following motorist: the driver is cooperating by merging partially across the median, allowing plenty of passing clearance, which in the cyclist's experience, is not uncommon on this road.
The diagram below shows the actual road dimensions at this point, with cyclist and cars drawn to scale. School buses use this route and it is clear that a vehicle of that width would have to merge across the median to safely pass a cyclist.
