Steam – Why did Steam erase all the games

Steam just erased all my games without warning. Here's my setup and what I've done recently that could be related (I'm including probably too much detail in the hopes it helps someone figure out what happened):

• I've had my Steam account in use on 3 computers for months, and added a fourth about a month ago. There were no issues with this other than the normal needing to coordinate which one is signed in at what time.

• Yesterday I restored part of a backup (made on Computer A several months ago) to Computer B. This was a bit problematic; it failed once, then again when I tried in Offline Mode, and worked the third time although there were connection issues and I had trouble getting Steam to exit after it finished. In the end this exercise was entirely useless since Steam wanted to download the entire games again anyways.¹

• Today I completed some of the downloads on Computer B (COD1 and Worms Crazy Golf).

• I then signed in on Computer C, forcing Computer B to be logged out.

• Then I put Steam into Offline Mode on Computer C, and logged in on Computer B again.

• Next I started up Worms Crazy Golf on B and exited Steam when I realized that B didn't have my saves since the game doesn't use Steam Cloud. (Despite this, Steam reported that it was syncing to Steam Cloud when I exited.)

• Finally I started Steam on Computer A before connecting to the internet, and it naturally gave me a connection error. I connected and hit Retry. Instead of logging in it said something like "Exiting", the main window flashed briefly, and it exited.

• I believe this is the point where it erased everything because the log-in window came back up about thirty seconds later and, while it remembered my username, it didn't remember my password and otherwise looked like it does when you start it up the very first time.

• After signing in none of my games were marked as installed, and my Favorites menu was empty (and ended up empty on the other PCs as well). My steamapps\common folder on the hard drive was empty and nothing could be recovered with undelete tools.

So, now I have a few questions:

• Why did Steam erase everything? Is this a known issue?
• Did I do something "wrong"? I was the one doing all of the actions, setting up and playing, though who knows what their heuristics thought of it. My account doesn't appear to have been suspended or anything.
• How can I prevent this from happening again?

_{¹The re-downloading could be because there were updates to the games, but I don't think so since the downloads were the size of the entire games. COD:MW2 couldn't possibly have had 11 GB of updates, right? Either way I am sticking to manual backups from now on, Steam's are too slow and obviously unreliable.}