It was Damon Kemp all along.
Damon Kemp, the man who betrayed Diamond Mine and cost Brutus and Julius Creed the NXT Tag Team Championship, further explained all of his betrayals during a sit-down interview on the September 20 episode of NXT.
Kemp explains that he even took a pay off from the D'Angelo family and that had it not been for Santos Escobar, it would have been a lot for the D’Angelos to become the new tag team champions.
Kemp also explained that when Roderick Strong found evidence to clear his name of the betrayal, he responded by breaking Roderick's phone and also sending Rodrick to the hospital.
There you have it, despite the tension between the Creed Brothers and Roderick Strong, it was Damon Kemp who was hurting Diamond Mine from the inside from day one.