Because the underwater topography focuses the energy:

http://www.triplicate.com/2009032810...-tsunami-prone

 

I saw some show on the Discovery or History/Science channel about some potential "megatsunami" not unlike the Crescent City tsunami, except its suppose to come from a lake. I think the idea is that a gient piece of earth/snow/ice breaks off a mountain, goes into a lake, shooting a wall of water in the opposite direction.
I think it's in here:

 

Not potential, this actually happened.