Folder Permissions Changing By Themselves?

The other day, I moved some files+folders outside of a folder just to organize things. I tried to put them back inside and I kept getting the "Cannot move file, it's being used.." error. I was pretty sure I wasn't using any of the files so I checked the permissions to find that under the security tab, inside the "Group or user name" was completely empty without any user names. I fixed it by adding my user name. This has happened before. A few months ago, I couldn't access any folders and files in this one particular folder without getting the "Acess denied" message. I found out that all my files and folders within that folder had no user names under the security tab which was a pain because I have over 100s of files in there and the only sure way to change the permission is to do them almost one by one.

I was wondering if there were any particular reason why it's doing this. I'm pretty sure that this computer is virus free and I almost never turn off the "Use simple file sharing" under the folder tool section. I've only played around with the permissions once over 5 months ago and I haven't touched it since. Also, is there a way to change every permission within one folder so that I won't have to do them manually one by one?