Excel Password spontaneously changing

I have just discovered that the password on an Excel 2008 spreadsheet I authored has become corrupted/changed in some way. Luckily, I have the same spreadsheet available in Time Machine from a few days ago, and can open it there with no problems using the original password. However, when I try either restoring it from there to my iMac or simply copying it as a new file, it will not open in its new destination and gives me the same password error message.

I am completely bewildered. Has anyone else come across this and is there a fix ?