I've had two Sour Sop trees for two years which I've kept inside during the winter months and outside during summer months. They are about 5' tall. I've had no problems until this year. This winter I added grow lights above them and they started growing and sprouting like they were on steroids. They are having trouble holding the new growth, the new leaves keep falling off. At every old leave, it looks like a new sprout is coming out.
Why Is Sour Sop Trees Dropping New Leaves?
Sounds like the extra light has them confused. Could be that they are used to going into a bit of dormancy, but the light from the grow lights is signaling that they should grow. Could also be that they don't have enough root structure to support the new leaves, or that they're not getting enough water. I'd try dispensing with the grow lights, and see what happens.