uploading giving funny results again?

I went through a period of 1-2 wks where I could not upload a csv produced by Excel 2013. It always errored out. I fixed it by opening the file with a text editor and resaving, then uploading this version. Oh well........

Today I am trying to upload a different csv produced by Excel 2013, it went into BI, but there is an extra column in BI that is not in my csv file. It's heading is "_Column" without the quotes.

All I did to produce this file is save a particular sheet (no formulas at all) as type csv.

What happened?

Where did this come from?
How do I prevent it?