Memory of Elephants - Agatha Christie

Hercule Poirot stood on the cliff overlooking the rocks below, the waves breaking on them.

Here...where he stands...the bodies of a man and his wife were found. Here, three weeks before that incident, a woman was sleepwalking and fell dead off the rocks.

Many years have passed since these events, so why does anyone seek to reveal the truth of what happened?

Why did these events happen?