I've noticed that if I have a document with a table in it and am doing MergeFields as well. Whenever I hit one of the buttons to change what the MergeField is showing (Field Codes, Preview, etc.). The ribbon UI immediately switches to the "Layout" tab for the table, even if I am not in the table. After you press the next button once for previewing MergeFields, the issue appears to go away, but to a client using this, it would be rather annoying.