The engineers said the findings raised questions about the design of
the levees and the testing of the relatively fragile soil during the
construction of the walls.

The likeliest chain of events was that water pushed sideways against the upper part of the walls during the storm surge and that the intense pressure caused the weak soil to give way, sliding the levee into the neighboring lawns. Had the pilings that supported the flood wall been driven deeper, into firmer soil it might have stayed put.

Read (New York Times)