I'd imagine it's because a player can easily bump and shake a goal post when jostling on the line, which you could reasonably interpret as accidental.Another one of those stupidly written AFL rules.
Why is "intention" written into the rule? Why isn't the rule just "if a player shakes the goal post it's a free against"? There is no reason whatsoever that this rule needs to be interpreted.
Write the rules clearly and unambiguously and this wouldn't be up for debate.
there is no reasonable excuse for leaping up when no one is around, though, obviously