Workbook_Activate not triggering

No, only if that window has focus. eg: if you had a browser open and your workbook is behind it.

Just try a test;

Private Sub Workbook_Activate()
    MsgBox "activated"
End Sub

I don't think so. Only if you actually activate the other one. I don't think it becomes active by default. Does it trigger if you click on something in the remaining workbook?

I am five versions past Excel 2003 so I can't test it.

Your title says Workbook_Activate but your post says Worksheet_Activate - which one do you mean? The former should be triggered, the latter not.

A Worksheet_Activate event should only be triggered by switching sheets in the same workbook. Switching between workbooks by any method should not trigger it.