Stephen Snyder found the answer at a talk by Sex At Dawn author Christopher Ryan:
Ryan explained that the group sex we’d just been watching had been prompted by someone throwing some apples into the midst of the group. The part about the apples didn’t make any sense, until he explained to us more about bonobo society.
Bonobos like apples. They like them a lot. As a matter of fact, it’s difficult to do bonobo research without a supply of green apples to motivate them to do the experiments. But they like group harmony most of all. And the sudden appearance of the apples in their midst immediately raises the threat of discord. Who will get to eat the apples?
If these were chimpanzees, the strongest males would immediately claim the fruit. There would be a fair amount of shoving, and possibly some bloodshed. But bonobos are so communal that the tension produced by something so precious as an apple in their midst must be dispelled by a gesture of community. In this case, everyone gets to cool off with a little sexual comfort from their neighbor. Then, self-interest replaced by a certain yummy group feeling, they settle down to share the apple.
(Hat tip: Charles Mudede)