It appears to compare checksums of your local files with the cloud and will re-download any invalid files again.
As to why the validation is happening automatically after updates download for these two games.
My Steam wants to validate TF2 at least every other day for some reason, which takes a good 5 to 10 minutes.
Do not run the verification process while performing other system-intensive processes, such as defragmenting your hard drive or scanning for viruses.
Do not run this process for multiple games at the same time.
I have 4 other games installed on Steam and all of them are able to successfully validate all files.
I've tried completely uninstalling and reinstalling the offending game to no avail.
Titles in GREEN are linked to the presentation materials.