What are you talking about here. You can download everything for TSC anytime providing you pay for it. If you mean sale items, I have checked my sale installer icon and nothing is showing today. Please explain what you mean?
Probably verifying your cache since Steam does that now. I deleted an ap' file out of a route to test something and steam redownloaded it almost instantaneously.
When I started Steam today there was something downloaded which was around 500-600 kb and I have not bought anything for TSC for some days!
It always does this. It may not even be downloading anything. It says it's updating if I recall, so it may just actually be checking, but some data will be exchanged no doubt.
Might be the cloud saves syncing. No changes are made to the game files. I could go on and get more technical - it could also be a verification thing that is triggered if the "StateFlags" in the appmanifest is not "4". This flag is set/read by Steam if there's a pending DLC download, a verification is triggered, all sorts of things. Again, I can't observe this here. If it were a download, it would be visible in the download manager which has to be manually cleared.
I can't detect any change to anything, so either it's just checking for updates, or as you say, syncing.
Checking for updates is a read-only process. First the "buildid" is read from the manifest. Then every DLC manifest id is checked, and an update triggered if any of these are out of date. Besides, we don't know what other games the OP has installed. It might not be related to TSC.
It's Steam pre caching shaders you can turn it off in Steam - Options. It's pre-caching gpu shaders the game might use, it's not pre-caching TSC shaders or any other game specific shaders.
