Can someone please tell me how and why a file corrupts?

Hope this helps:

Windows is constantly adding, removing and modifying files in the registry and eventually some of those files may get corrupted in the process. When a registry file gets corrupted, it lowers the performance of your PC. The more corrupted files in your registry, the more your PC’s performance diminishes.

Another reason why the Windows registry may become corrupted is during the installation process. Whenever you install a new program, the registry is accessed and changes are made. However, if something goes wrong during the installation process, such as if your computer shuts down while the program is installing or if there is an under lying technical problem, the program you are trying to install may not work correctly and your Windows registry may get corrupted. If you install a new program, make sure that the program you are installing is reliable and that it comes from a trustworthy publisher. A side from potentially getting viruses and other malware, installing a program that’s questionable can also potentially corrupt your registry.

You can also corrupt your Windows registry during the uninstall process. When you uninstall a program from your computer, you are telling Windows to remove the program’s registry files as well as the program. But if something goes wrong during the uninstallation process, your registry can be negative affected. When Windows wants to delete a registry entry, as known as ‘keys’, it marks it so that Windows will no longer see it. It never actually gets removed from your computer, the keys just gets marked as deleted. Whenever a registry file that gets marked as deleted and interferes with a registry file that is still active, then the Windows registry will get corrupted. The best way to prevent your registry from getting corrupted during the uninstallation process is to follow the recommended steps for uninstalling a program.