Because of the sudden disappearance (his last posts don't look like someone who's about to leave, I always assumed something bad must have happened. The best scenario I can think of would be jail, the worst scenario would be either an accident, or killed.
If Satoshi is a group of people, the sudden disappearance because of an accident or jail is much less likely. In that case "in hiding" would be a possible scenario, but I don't think Bitcoin was such a big deal back then so that doesn't make sense either.
Satoshi; whether a group of people or a singular person likely has a life outside of Bitcoin, and being in hiding doesn't necessarily mean its related to Bitcoin.
Considering how careful Satoshi was to remain anonymous, it's quite likely that there wasn't really a "disappearance" of some particular entity. Someone, who could be otherwise well known (but definitely not CSW) and perhaps remains very much engaged in Bitcoin, decided to stop using that pseudonym, which was created for the purpose of kick-starting the project and was no longer needed when the project picked up speed.
And, we've had several people over the last decade get accused of being Satoshi including a few developers. If my memory serves me correctly I believe at one point Gavin was accused of being Satoshi, although lets be honest Gavin got accused of a lot of things after going to the FBI to talk them through Bitcoin.