In reflecting on my M. nigra suffering from extended dormancy, I believe that the one episode of drought damage back in September is the culprit.
I am quoting this from the web article link below:
"Growing periods with little water can lead to decreased rates of diameter and height growth, poor
resistance to other stresses, disruption of food production and distribution, and changes to the timing and rate of physiological processes, like fruit production and dormancy."
Back then, my tree was still potted and in full sun, and on one hot afternoon, and without a warning, it looked almost completely dry. Unlike the thin fig leaves which will droop and wilt when the tree goes dry, giving a warning sign of impending damage before it happens, the thick leaves of nigras will not wilt but instead, will directly dry out. In trying to avoid overwatering it for fear of root rot, I ended up underwatering it. At the time, and to my great relief, I thought it had recovered. It had lost almost 80% of its leaves, but managed to grow a few new ones before it went into dormancy. In the fall, I planted it in the ground.
I am hoping this will serve as a precautionary warning to others, especially those with potted nigras, to make sure they don't go dry.