Because it's a so called twilight launch. The sun is just behind the horizon, already set for observers on ground level, but not yet set for obersvers (or objects) high above the ground. Thus, the exhaust plume of the rocket and all the gasses in it (and additional condensation from pressure areas I suppose) are being backlit and illuminated.
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u/NUMBerONEisFIRST 2h ago
Is that advanced propulsion or does it normally look this spectacular?
I've seen a lot of launch videos but this one is way different.