Airliner headlights flashing alternately
Some modern landing lights have a pulsating or alternating mode. It's just for being recognized more easily, has nothing to do with an emergency.

A constant white light heading right for you over a city full of lights doesn't get as much attention as a pulsating light which gets bigger each time as you get closer :)

Here is one example: https://www.preciseflight.com/pulselite-pulsing-aircraft-lights/

and their video: