Switching between modes IS a design change. Each layout has a default view which is the last view specified in layout mode. Since your file is served and you have design rights, and since you changed the view, you are being asked if you want to keep it as the new default for EVERYONE when they switch to that layout. :-)
This makes sense, but I'm still a little confused as to why the preference option "Save layout changes automatically (do not ask)" doesn't suppress this dialog box. Also, I swear it didn't always do this.
One last thing.. If I download the file locally and open it, I am able to switch layout modes without any such prompt. Why only when accessing it using the server?
You make a good point but all I can tell you is the way it works, not FM's thinking behind it. When served, you may NOT want to automatically save your changes, such as when you switch from form to table layout (to view the raw data). This is only a guess of their thinking.