It’s winging its way westward, spawning reports of Thunderbird sightings as it goes :-p

As to speculation as to what spooked the animal enough to make it run over a cliff, it really could have been a whole range of things. A sudden noise, a predator, the flies Kittenz mentioned, parasites such as a certain type of tick known to heavily infest moose and make them a little bonkers, a car horn from the road itself (as has been mentioned), among others. There is a whole a myriad of possibilities for what could have spooked it, and it’s probably something we’ll never know for sure in this case. The moose didn’t necessarily go charing out over the edge either. Due to the steepness of the rock face, the moose could have been startled and simply lost its footing on the terrain.

Late Spring and Summer is dangerous because the mothers are caring for their calves, and Fall is dangerous due to mating season, with bull moose being aggressive due to the reasons I mentioned in my comment above. As I said, mating season is actually a cause for a good deal of unprovoked attacks on humans, on par with mother moose protecting their young, I would say.

Moose can also be dangerous when they are spooked, or during winter months when food is scarce and they are hungry and irritable. There are many reasons why they can become aggressive towards humans and they are certainly an unpredictable animal that needs to be treated with the respect any 1500 pound animal should be given.

FYI, Mystery Man, the danger to humans from moose, which as you indicate is considerable; causing more human harm over the course of a year due to tramplings and auto-collisions, has more to do with the protective instinct behaviors of mother moose guarding their very vulnerable calves than with mating behaviors, though they can be unpredictable at any time and one should not be fooled by the docile look in their eyes.

If there is, though, I haven’t read it yet. Maybe a guy with a tow truck was the sole witness.