Pacman keeps a local cache of the packages that exist, so it knows that whatever you were trying to install has a version X and that depends on library Y on version Z. Except when it tried to download the library Y on version Z the package didn't existed anymore, most likely because a new version W was released since you generated your cache. The y option updates the local database first, so it knows the proper up to date versions of things.

That being said running -Sy is a bad idea, because for example what you're trying to install might update a library that the system expects to be an old version and then everything breaks. If this ever happens again the correct way to fix it is to first do a system update -Syu and afterwards install the new package.