My buddies snomobile (600 HO Skidoo Rev) actually has reverse by the motor running backwards.
When you hit the reverse button, it slows the engine down to a stall almost, fires it backwards for reverse.
Kinda cool if you think about it.
As said, it probably just kicked back and fired. I'm sure you didn't hurt a thing...other than your melon scratching it while sitting there thinking WTF??? LOL.