#1 The Planet's Champions cheat to win
In order to retain their titles at any cost, Bryan and/or Rowan might just do something behind the referee's back in order to pick up a cheap win. Bryan did it when he won the WWE Title against AJ Styles, and he's resorted to post-match beat-downs of opponents regularly.
There might be a point in the match where the beef of his opponents becomes too much. 'The Killer Vegan' then finds a way to hit another low blow or hit either Otis or Tucker with a chair or something else. Extreme Rules is the next PPV, so if they cheat with a foreign object, it might be the impetus for another match between the two teams next month.
Rowan also has attacked people when the ref's back was turned, but since he's a legal competitor this time, it wouldn't be seen as interfering. There will likely be some 'big guy posturing' during the match in order for each big man to assert their dominance.
Perhaps Rowan and Bryan show their dominance by tricking the new boys on the block into a trap in order to retain the titles. Heavy Machinery winning the titles would only be in a clean fashion, but there are several situations where they could lose. Getting cheated by the champs is one of them.