In the wild, when Elephants have an itch they can’t exactly reach with the trunk, they find the nearest stationary object scratch on.  
In this case Obi the Elephant found a random Acacia Tree and you know the rest.
 
Zeka the Giraffe and Lesotho the Rhino saw how many leaves where being shaken off by the butt scratching and decided to take this opportunity to eat as many of the leaves as possible. (Even though Zeka can already easily get them.)
 
Lesotho being a grazer and all, really enjoyed the unfamiliar, but delicious taste of the leaves that fell from the tree because of Obi’s butt scratching. Then she started looking at Obi’s butt for some reason…..
 
As Steve Irwin would say, “Crikey! That is some unuoosual animal behavyah……An I Loike Et!”