We need more information on this if possible.
Do you have ANY custom rules setup in Groupy under Grouping rules? If so what are those rules as it could be there is a rule set which matches a lot more things than you might be wanting.
When it happens it would be extremely useful if you could click on the Groupy dropdown to the left of the tabs whilst holding down the CTRL key. Then go to the debug menu item and take a screenshot of the submenu and post it here. This will give us a clue as to why it decided to group that window.