[Man-of-the-Wold]

Connected to what I couldn't find the point. Sauron feinted away from Dol Guldor at the time of "The Hobbit," the Balrog's presence had been felt by Dain at the Battle of the Gates of Moria roughly a hundred years earlier.

Balin and Co. did not try to recolonize Moria for many years after the Battle of Five Armies, and Sauron's setting up shop at Barad-dur.

It's hard to say how the Balrog reacted to there presence, whether he was asleep and took no notice, or if he waited until there were more orcs again to help chase out the Dwarves. Seemingly, the Orcs were not too plentiful in Moria when Balin entered, which is reasonable, and even when the Fellowship passed through I doubt it was anything like the multitude depicted in the Film.

So, perhaps more goblins appeared from the far deeps or were let in through secred ways known to them, with the Balrog's assistance, or perhaps at Sauron's behest, too. Nevertheless, they reappeared in numbers that the Dwarves could not fight off in association with the Balrog, which seems to have chased out the Dwarves orignally not through confrontation as the master of Orc bands, but by being a nameless terror in the Mines over many years. He probably didn't want to repeat that tedious exercise.

In any case, it seems best to see the Balrog not as servant of Sauron, but as a Free Agent with common interests, who in all likelihood would have exploited the War of the Rings.

Finally, with the exception of Gothmog, I think Sauron clearly outranked the other Balrogs. They were Fire Spirits, not the type of high-ranking Maia like Osse, Eonwe, Melian, Olorin or Gorthaur the Cruel who was Morgoth's greatest servant and mighty in the lore of Aule, to paraphrase the Silm.