Everytime I opened certain Spreadsheet, excel opens another existing spreadsheet

There is a weird behavior in Excel in 365 2016.   Everytime I opened any of two spreadsheeets I have, it also opened a second spreadsheet which is also stored in my harddrive.   This bothered me for a long while as I could not find any answers to the problem.

While trying to resolve the problem, I opened a spreadsheet that did not have the problem and then the problem with the other two spreadsheets went away. 

There must be a problem in 365 excel as I had not seen this problem in previous excel (non 365).

It made me waste a lot of precious time.