Once it started, there was just no stopping it.

Bombing a hill, while fun, opens you up to a whole bunch of potential problems. First, you could lose control, causing you to fall off your board and tumble wherever the inertia takes you. Second, someone or something could suddenly get in your way, causing you to slam into it and for each of your limbs to go flying in a separate direction while (hopefully) staying attached to your body.

In this case, one person lost control and crashed — which caused another to lose control, then another, then a third until the whole trip was ruined.