There are wire loops embedded in cuts in the road in each lane at traffic lights.
These loops detects your vehicle when you stop over them and then signal the lights to change.
If you stop past these loops or before them, the grey control box will not receive notification to change the traffic lights as it hasn’t detected a vehicle is waiting in that lane to turn.
A great example of this is where the first car stops on the pedestrian crossing and the second car keeps far enough away from the first car and neither are over the wire loops. They will never get a change of lights whilst there is no vehicle over the wire loops.