As implied, Excel is recognizing the field as a date field and is autoconverting it to store it as a date internally, and then putting it in the default date format.

The first thing that comes to mind is seeing if you can change Excel's default date format to match what you want. I don't know if there's any such feature, but it might be in some internationalization options.

The second thing that comes to mind is "why are you using Excel?" It sounds like you've got an automated process (since you imply that these files are not being opened in Excel by hand). Maybe you could find a different tool than Excel to do whatever it is that's being done to the file. If you could fill us in on that part, maybe we could help figuring out what that tool might be.