Why would opening the SumInfo window in one file cause other files to close?

I am getting a "Save modified documents" window when I open the Summary Information window to change Custom properties. It happens randomly.

"One or more documents being closed as a result of this operation have been modified." ...Why would opening the SumInfo window in one file cause other files to close?

If I save or don't save, will some documents actually close? Which documents? Argh! Which option do I choose?

SolidworksGeneral