Second wave was a surprise.
A lot of people were unaware that the first wave would not be the last one, and went to check things out after they thought it was safe. Heck, how many of us knew before this that such waves came in groups?
The first I ever heard about the "receding ocean" warning was when I read "Childhood's End" as a kid, and there was a scene where this happened.
Oddly, animals seem to be smarter about this sort of thing.