A broken transmission. Can you reverse? If no, there you have it. If you can reverse, then the only other possibility is a slipping belt.
Of course, I'm assuming that you aren't really asking why the mower rolled away unattended whilst you were having lunch? If that's the case, then the answer is that you must set the parking brake WITH THE ENGINE RUNNING on variator-equipped models. If you don't, it's possible to get a condition whereby the drive is de-clutched (so the engine can't act as a brake), but the brake is not on (even though the lever is at PARK).
Hope it didn't damage whatever it hit :)
phil_saunders(a)bigfoot.com
Phil Saunders
June 2012
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