Simple answer is light travels faster than sound.
I live near an airport so planes are usually close enough to the ground that I can hear them before the actually cross over. That is a commercial airliner flying somewhere between 500-600 mph I think. A plane at 30k feet plus seems silent somehow.
Military jets can fly faster than the speed of sound so naturally the sound seems to lag. It’s kinda neat imo.
Light takes tens of microseconds to get to you, while sound takes tens of seconds to get to you. So, when you look up, you see the plane as it was microseconds ago (basically now), but the sound it emits is still way above you – the sound from 30 seconds ago is just now getting to you, making it seem like it’s coming from behind the plane.
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