Q.Premature Mulberry
Last year I grew some mulberry cuttings all these produced were leaf every few inches. July I pruned them back. Around first of August four sections started to produce flowers then Berries. 5 days ago four just fell off and today 5 fell. Watering is good, could be weather. It’s been unusually hot here in San Diego.

Certified GKH Gardening Expert
There are several possibilities including weather, poor pollination, overbearing, and insects. Here is more:
https://www.gardeningknowhow.com/ornamental/trees/fruitless-mulberry/mulberry-fruit-drop.htm