If you charge an elephant in the flank or rear, you receive combat bonuses but also run the risk of possibly being stampeded. On the other hand, if you charge the elephant's front, you receive no bonus for flank/rear attacks, but you also have no risk of being stampeded. this creates an interesting risk/reward decision that would not exist if ya unit could simply charge the elephant from any side without ever having to risk being stampeded itself.
It is also possible that Martin may one day decide to issue an erratum declaring that an elephant stampede should be based on the direction from which the beast was attacked, as you have suggested. I would be content in either case.
"Experience is the teacher of all things."
~ Gaius Julius Caesar