I've noticed this with two games:
Both were free and both download updates incessantly, so I uninstalled them.
I've been aware of the validate steam cache files for a while. You can do it manually from the properties tab by right clicking in the game list. If something goes whacky with your game, this sometimes fixes it. Steam support suggested it once and it got me out of a problem I had, so I try it again on occasion. 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. I don't know, it doesn't make much sense, since the download just happened and should not have been corrupt. But I agree, it is annoying.