I'm not an expert in landslides, but it appears to me that such things are rarely symmetrical. Note also that the loose rock blocks the only path to Melenkurion Skyweir and is in such a place that intruders would have to walk downhill on loose rocks.Great, scalloped wounds stood opposite each other high in the walls...
So, who do you think blocked the path? The means are commonly available among powerful magic-users. The window of opportunity reaches from after Amok's making in High Lord Kevin's time to "not very recently". The motive is obscure beyong the intent to block the path. New Lords are out due to lack of information.
My best guess is that the place used to have a Doom's Retreat style Word of Warning put there by Kevin in order to protect the Seventh Ward. We know that Kevin could make highly durable standing spells. During the centuries someone, possibly a Haruchai, stumbled into it and triggered the avalanche.