excel skips to another workbook for no apparent reason

No, I didn't.  There were a couple of decent suggestions earlier in the string, but I haven't tried any of them.  I sure wish there was a simple remedy, as it is driving me crazy!

Let me just say, again, that we were completely stuck with this problem for years, but the registry change has been a 100% solution for us. We don't have to do anything funny when opening files, and everything works pretty much the way it is supposed to. We have other Excel anomalies of course. But this one is really worth trying. There may be multiple issues involved -- other people may be stepping on different flavors of the bug -- but for us we went from 100% pain in the butt to 100% not an issue.

Not sure if this is the issue or not, but it might be related to the provided add-ins - analyst tool pack, EURO tool or the Solver. Reason for suggesting this is that I had the situation where I had no workbooks open - i.e. start Excel - open a blank sheet, then file-close. I then open the VBE and I noticed that periodically I saw the selected VBA module change to one of the add-ins, and then focus was transferred to be blank excel.

Now I have not had time to test this or been able to replicate it. However it might be an idea to try turning off the microsoft provided add-ins and see if this improves the situation.

I have a little bit to add to this.

I have the exact same problem, but I originally observed it just a little bit differently.  I noticed that Excel workbooks frequently do not respond until I click into a different workbook first and then back into the one that I am editing.  If I do click into the cell, enter data, and hit enter, I get the same results as the OP.

I eventually realized that Excel often "thinks" that I am in a different workbook because 1) If I print when this problem occurs, it shows the print preview of a different workbook than the one I am editing and prints the wrong workbook and 2) if I try to save when this happens, it saves the wrong workbook.

I am guessing that this would be the same for everyone here reporting the issue in this thread.  I don't know if this gets us anywhere since we may have already assumed that it is an issue with the active workbook, but I thought I would share since I did not see anyone else mention these two symptoms.

Unfortunately the workarounds are not really ideal in my situation, so I am really hoping for a solution.

I have this issue several times per hour. It appears to be an issue that has been reported for over 3 years. 

I see the solution of using a registry edit create separate instances of Excel, but I also see that this solution only works when you use the file explorer from within Excel. When your files are 6-7 layers deep in the server this is a big time-waster. 

Microsoft, why is this still an issue 3 years later? What are you doing with your time and our money that's more important than keeping one of your core pieces of business software running properly?

That's good to hear. Glad it worked out for you!