I listed Tsunamis because apparently they are usually caused by huge amounts of land sliding into the sea, or even under the sea. So I suppose I was picturing a big hunk of IceWall sliding into the sea.
It DOES make more sense for only a small part of the wall to fall, but breaching a wall of ice at 50-100 feet thick would take something pretty incredible.
What if the horn wasn't actually a weapon but really for calling something. Something big enough to punch through? It all comes back to scale...
I mean, if it's 800 feet tall, something must have REALLY scared Bran the Builder and everyone enough to get working on it for thousands of years, because Others, while scary, are still human sized and can be killed easily if you have dragonglass.
Nan's stories make it clear that the Others are near-unstoppable without magical aid. Certainly as the temperature drops they will get stronger and stronger.
I'm fairly sure the wall is not so much of a physical than a magical barrier; certainly someone commented that there are spells built into the wall to bar entry into the world of men. If the horn sunders the spell, then the wall is just a mass of ice...and who knows? Maybe the others will kill all the others and make a zombie pyramid to climb up!