Years ago, I interviewed the now late George Headington, who was the CMO in Salida during the 1950's. He told me that 485, which had just been overhauled at the time it fell in the pit likely had a leaky throttle (not uncommon) and "slept-walked" into the pit when a hostler forgot to put a chain under her drivers. There actually were a lot of near new parts on her from her overhaul, and Headington said that many of those were used when he and his crew overhauled 489--the last locomotive overhauled in Salida before the shop was closed.
From what I was told in Chama, when they overhauled 489 to put her back in service, they found quite a few parts numbered "485" on her at that time, so Headington's story checked out.
Interestingly, George Headington went on to run the D&RGW's shop in Grand Junction for many years. His son ran a oilfield supply business in Grand Junction, and as part of that wholesaled a lot of the lubricants used both on the D&S and C&TS for many years. Headingtons also restored classic and antique cars for a hobby, and had probably one of the largest collections of restored Hudsons anywhere. Interesting folks.