Excel often save .csv files as tab delimited format. What happened?

Just to expand on Chris76786777's answer on this possible Excel bug, I have also noticed if you open a CSV file that is encoded with UTF-8 with a BOM header on it (UTF-8-BOM), add some columns, and simply save it, it will save it with tabs instead of commas as the delimiter, but you will not see the (corrupted) results until you close the file and open it again in Excel.

If you open a plain UTF-8 CSV file without a BOM then the same edit above works just fine and you can reopen it in Excel normally.

One way to work around this is to open the CSV file in a robust text editor like Notepad++ or Vim, and convert the file encoding from UTF-8 BOM to just UTF-8 before fiddling with it in Excel. (I realize the BOM might be needed sometimes for Excel to understand different languages properly, at least according to this.) If you can remove the BOM, hopefully your results will then be more predictable (at least until Microsoft finds that this behavior is a bit odd.)

When you double-click CSV file and it opens in Excel what actually happens is that Excel treats it as a text file, however it's being converted to columns "on the fly". It's still text file, though.

In order to open CSV as CSV in excel and then save it as CSV you need to import it.

Unfortunately the default action of importing CSV from double-click is gone with Office 2003.

For you it's either to get into the habit of importing your CSVs to Excel always (has it's advantages if you have large numbers - 12 digits or more) or switch to other tools - OpenOffice for example.