Rape

It's a bit hard to make out because these koalas are so far away.

The one that's higher up is the male, and he is vigorously chasing the female in attempt to mate.  Male koalas can be very aggressive at this time of year, and the females do all they can to get away.  The males grunt loudly like pigs and the females scream with fear.

We didn't hang around to see what happened - it's a bit distressing.  Not a lot we can do about it anyway.  It's likely the female fell off the branch.  If she did it would have been a fairly soft landing.