Why do birds gather on telephone wires at dusk?

Question

and then fly backwards and forwards in formation?

Answer 1

Basically, it is a phenomenon called "roosting". They gather at a place and communicate among themselves. They narrate about the events of the day and prepare for the night.

Answer 2

Because birds have a sense organ for magnetic fields, and power wires have strong local magnetic fields.

They fly out and back because they're waiting for someone to decide it's time to go; when they have a consensus, they get moving.