Why do hummingbirds fly backwards?

Hummingbirds have the ability to fly up and down, and forward and backwards because they have a rotator cuff (a supporting and strengthening structure of the joint) which allows them to move their wings in a figure eight motion. In contrast, other birds do not have rotator cuff and can only fly up and down.

They do this when feeding on nectar from flowers.
They fly backwards so that they can remove their beak (or bill, whatever you call it on a hummingbird) from the flower while in flight.