Optimizing cache deletes all my files and makes me download the update again?

[MasqueradeP]

I'm confused on why my game files got deleted after I optimized my cache and that took place after downloading the updates. There was no error, the game recommended that I should optimize my cache for faster loading in game but after I did that, I suddenly had to download the game starting at 13gb. Can anyone explain this if possible? I'll not be optimizing my cache after I get it done downloading next time.