By 'topple', we usually mean that something fell to the side, not straight down. Collapse can mean breaking, then falling straight down or to the side. A roof would probably collapse, falling straight through to the floor or ground below, but we wouldn't say it toppled. In the case of a tower or tree which was blown over by the wind or was cut down, we would more likely say they toppled or toppled over. You can also say that a government or leader can be toppled if they are overthrown.

the only difference is: a pile of books can topple, but a person does not topple. a person can collapse. topple is just for something that is stacked (books, building (bricks), plates, boxes). does that make sense? :)