Amongst other ways they are over topped or a sand boil develops on the land side. The sand boil, is formed by the water finding a path in the ground under the levee to the land side. This creates flowing water that washes out a larger path. They usually ring the sand boil with sand bags up to a level where the water stops flowing which can be as high as the level of the water in the river.
The bank can be eroded away on the outside of a curve in the river too.