Well I know I've heard it said dozens of times in the Balrog discussions that the Balrog of Moria fled after the battle that ended the first age (my copy of SIL is on another continent, and I'm afraid I can't pull the correct quotes and names off the top of my head) and "hid" in the depths of Moria. I really don't see how he could be "imprisoned" there. As you said, Gandalf barely managed to dispatch him; his sorcery could match Gandalf's, as the breaking of the door represented; unless the Valar themselves put some kind of geis on the Balrog to prevent him from leaving Moria (which is probably not true, because he battled Gandalf outside on the peak), I don't see how the Balrog could be "kept" or "imprisoned" in Moria.

I doubt he was awaiting the rise of Sauron, because he sure didn't help the Dark Lord when Isuldur and Gil-Galad were chopping his fingers off.

The Balrog might have chosen to stay there for reasons too obscure to be understood or represented by the narrator. However, I suspect the truth may be that he had "fallen asleep," or become accustomed to isolation, until he was disturbed by the dwarves mining too deeply. Then, like a cranky bear discovered in its hibernation, he attacked them, then couldn't fall asleep again. As for the orcs, I don't see a particularly high level of interaction between the Balrog and the orcs in the text; my impression is that the orcs did their own thing and kowtowed to the Balrog whenever he became active, as he did against the Fellowship.