Same here, have only one bush and bearing fruits, my guess is plum
pollination.

Konrad

I would suspect that they may be self fruitful or easily pollinated by plums or other cherries. I have one that is a mile from any other nankings (and half a mile from plums or cherries) and they typically are the first thing to bloom in the spring and mostly finished when the other stone fruits start coming out, yet it is loaded every year with fruit.

I didn't have any fruits the first 6 years or so until I put in some plums,
wild cherries have been around all the time.

Konrad

Hansen bush cherry pollinates with plums but it is not related directly to Nanking.