It's covered in the stampeding movement on page 40 of the main rulebook.
""Once it has started to stampede it continues stampeding in the same direction for the rest of the game or until it leaves the table".
Any charge against multiple targets must be made by moving straight ahead. In this example the Triari declare a charge against both the skirmishers and the elephant.
The skirmishers must evade and the triari contact the elephant.