I will third or fourth the "wanna-be pro cyclist on the trail" peeve. I am a former cyclist. I would have avoided MUTs at all costs, and if forced upon them, them it would be at a leisurely pace.
Every summer, typically July, typically during the Tour de France, a bunch of Freds will appear on the trails going at a pace that is too fast for the trail, and in fact you can see they are at their limit of both speed and control.
They're usually bombing around blind corners and blowing past from behind with no warning or word. No helmets. Reflectors on their wheels.