The camera's resolution is too low to see what might be the cause.
Gravel would not seem to be a factor on that type of road. For going straight on a flat road, it seems too wild for a simple blowout or gravel--and grates don't seem to be involved (again, very low quality video).
I'll guess at a broken cleat or pedal--in the aftermath it seems he'd hurt his testicles, and so came down hard on the saddle.
The second/following cyclist seems to stop pedaling and swerves left at the same time the lead guy loses it, so there must have been something he saw or heard. There's no lag, he knows at the same time as the lead guy that something has happened.
The lead guy goes over his bars, so he may have had time to hit the brakes, initially hoping to save it. Or not. The bike was slowing, and the rider's body was not, so maybe just naturally over the bars.
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Mike--if the truck had killed the guy and I was a lay jurist, a video like this would be extremely valuable. Tho there could be some over-riding Japanese formula for figuring out liability, from my POV this would not have been the fault of the following driver.
Finally, seeing this, if I were the truck driver, I'd be advocating for better quality drive recorder cameras asap.