excel skips to another workbook for no apparent reason

This solution seems to have resolved the problem for me. I have not had a recurrence of this issue since running each instance of Excel in a separate process. 

So, this is a very helpful tip. Additionally, I deleted the default Excel shortcut in my task bar and replaced with the shortcut that contains the /x switch.

I think, though, that it takes away from the point of multi-document-interface in that Excel has a tougher time communicating with itself on cut-and-paste options (that I've noticed). So, MS needs to resolve the issue. It can be a VERY critical issue for those users and companies that depend on Excel (and what company doesn't, at least to some extent).

Thank you JonathanKung

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

This is more of a workaround to the problem. Unfortunately I don't have an actual fix. Isadore's link below is also very useful if you prefer not using the run -> Excel /x switch each time to open up a new workbook.

https://support.microsoft.com/en-us/help/3165211/how-to-force-excel-to-open-in-a-new-instance-by-default

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?

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. 

We have been using the registry change and the basic symptom has disappeared. I don't know what you're referring to about using file explorer from within Excel. This seems to have affected every Excel instance however launched. We no longer flip from file to file, although there appear to be some other side-effects (presumably, related to things that Excel was able to do with multiple sheets in the same instance). 

I agree that the lack of any apparent action or acknowledgment in this flagship product is frustrating. But it seems pretty much par for the course in the software biz today.