Then I thought, I bet this has something to do with deer. So I looked it up, and the National Park Service helpfully explains the connection:
In recent decades, naturalists have noted the expansion of pawpaw from well-drained, lowland habitats into drier, upland forests. This phenomenon appears to be driven, at least in part, by patterns of deer browse. Deer find pawpaw foliage unpalatable and, therefore, avoid browsing pawpaw seedlings and saplings. Instead, they preferentially browse species such as spicebush (Lindera benzoin), oaks (Quercus spp.), red maple (Acer rubrum), and blackgum (Nyssa sylvatica). Deer avoidance of pawpaw is evident in NCRN forest data. . . .
So, yes, the spread of pawpaws from moist, sheltered spots along rivers to dry uplands is all about what deer will and won't eat. When I was young there were few deer in the forests I frequented, so there weren't many pawpaws.
Deer are radically reshaping the forests of temperate North America, and if we don't get serious about killing them we will end up with strange, impoverished woodlands. Pawpaw trees are not tall, with a maximum height of 50 feet, but in some areas they may end up taking over from taller trees:
If deer populations remain high, the forest canopy height may decline over time, particularly in areas where pawpaw is the only understory species available to replace dead or dying canopy trees. Or, perhaps the forest canopy would become patchier, with short patches dominated by pawpaw and tall patches dominated by other species that are represented in the sapling layer of the forest (American beech, for example, is deer-browse resistant and the second most common sapling in NCR forests).Interestingly, a similar phenomenon has been observed in over-browsed forests in central Pennsylvania. In these forests, the small, understory species striped-maple (Acer pensylvanicum) has become increasingly common in the forest understory over a 60-year observation period. At the same time, tree species that are capable of growing into the forest canopy have declined by 85%. Striped maple and American beech were found to make up 82% of all trees in the deer-browsed forests.
Forest that consist entirely of beeches and pawpaws. . . .