There are a few possibilities as to why a file can become corrupted.
Hard disk corruption
The most common cause would be that the harddisk has a bad sector. The more a system file is being read to, the more the state of a disk is read which simply causes wear. Now, it would take millions of reads before it could cause a bad sector, but it could happen.
A system crash while the file is opened for read and write could cause this problem too. Although most of the time, the file will be opened for read only, sometimes for unknown reasons, the file is being opened for read and write (usually to make sure the file is opened and a lock to the file is claimed). If a crash happens while this file was opened in read/write mode, it may lead to corruption.
Program error while updating
It is possible the file got corrupted while downloading it and then during the installation of the update it got replaced with a bad copy.
It is possible a dll file is shared and there are more than one versions available. One program could expect an older version while another updated it to a newer version. The other program could claim the file to be broken somehow.