I'd guess it was changed when his Q had been changed to do physical damage and the active give him attack speed rather than it being passive and the new passive is 15% of his total AD added in every AA.
Patch 3.7, they had mentioned them making his Q pressing to be for more than just the first 5 seconds so it is possible that they had changed the ability entirely and when fixing that, they had forgotten that it originally was pressed then you'd press the E after the Q and do a stun+rend at the same time. Likely oversighted since the Q change was such a huge buff that Udyr was actually played Top for a while there with an early game that made pre-nerf Rivens cry in a corner. Literally. Udyr with ArP runes going Tiger Udyr would sit in Bear form AA'ing minions and if an enemy without armor items would get near, you'd AA for the stun, switch to Tiger and half life them with the Q alone. It's so powerful that you can start Q and solo any camp by pressing Q at the start of the game, AA the minion, then press Q for the AA buff on top of the 2nd shot of your Tiger claws, similar to what you'd do with Phoenix Udyr except with Tiger, you could kill a blue in about 4 seconds with 15 AD in your runes without smite.
Tiger Udyr was scary as hell after they had buffed Q, so for them to do a minor oversight like a Q+E combo? It's no surprise. It would be OP as hell given how much damage that Q does.