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They'd usually say; "No, I didn't do ANYTHING at all".... but they (sometimes unknowingly) did.
Did you do anything?
One thing comes to mind...
If you had 755 files in place, deleted them, the uploaded new copies, the 755 would not be preserved (as it would be if you had simply uploaded new copies of same-named files)...
Again --- Did you do ANYTHING to the files, the directory, etc...?
(1) as Lexipixel already suggested, you did it yourself, inadvertently perhaps, as a side effect of running other stuff
(2) somebody else did. In that case, perhaps ...
(2a) your system admin did (as a first step before discussing your mailing action and terminating your account due to too many complaints received)
(2b) another user of the server did playing games, if the (shared?) hosting environment is somehow insecure,
(2c) a hacker from remote did, if your hosting environment and/or some user scripts (your own two, or even others of other users on a shared server) are weak and exploitable.
Only you and your hoster/admin can look at the logs and the complete environment and investigate further.
I did not do it deliberatly and members know they will get email from me admin when they sign up to my program so I doubt it would be a spam complaint.
I do have shared hosting so perhaps a hacker did change it. I will check with my hosting company to have a look at the logs.