I guess it's pretty clear that the only way the locomotive could have ended up that far into the pit is for it to have fallen off the side of the turntable, as opposed to heading directly into the pit from the periphery.
If the turntable had been lined up and locked down in line with the lead that the engine was parked on and the engine walked forward due to a leaking throttle, it seems likely to me it would have simply walked to the other side of the pit and ran aground (assuming there was no companion track on the other side of the pit) and wouldn't have gone very far after the pony and one or two sets of drivers hit the dirt.
Being Salida, this must have been a three-rail turntable, thus considerably wider that the turntable at Durango, for instance. If the turntable had been lined up to the next lead over from where 485 began her ill-fated final run, I suppose it's possible that she could have walked out over the turntable decking and got well out onto the table before angling off to the side and into the pit. I would love to have been a fly on the wall of the pit that day.