I don't know how it ended up in the ditch, but my thoughts are:
1) Hit (or nudged/pushed) by another vehicle
2) Bored kids pushed it by hand
3) Someone tried to steal it, and lost control
Although any of the above scenarios are entirely possible, and only a fairly significant impact for option 1 or a piss-poor thief for option 3 would leave marks, the burden of proof lies with the insured - ie, if there isn't a significant body of evidence that point to tampering (ie punched in nose, smashed window/broken door lock, etc) they're going to assume that it's just as possible for me to have parked it without the handbrake or putting it in gear, and it rolled in when I closed the door.
The assessment guy I talked to even commented on how easy it would be to slim jim/bump without leaving damage, but there's nothing I can do to \"prove\" that failing testimony from an eyewitness, etc.