[Galadriel55]

I agree with Rumil. I think that the Balrog went into hybernation for the larger part of the 2 epoch. I think that it was waken by Sauron's willpower when Sauron was gaining strength to fight with Gil-Galad and Elendil. I think that he kind of summoned, and if not, then simply "awoke" all the evil that he could possibly get working. Sauron's first step was to become the master of all evil in M-e, so that he would overpower the good aries. However, the Balrog did not obey his will, and my reasoning for that is Balrog is almost as powerful as Sauron - he, as well as all the Balrogs, were once maiar. Sauron was the most powerful maia under Morgoth's rule (according to the Silmarillion), but I guess that Balrog comes very close. So, instead of joining Sauron's army, he just woke up.