I don't find it too weird. He was under extreme emotional stress. So he had no subconscious breaks. And by the time he got to his dad he hit the gas.
I dunno it would like SUgarBoy lifting a boulder or McBoom exploding his room in their first activation. I always thought it was clear that quirk gradually grows from barely doing anything to possibly being OP not that you didn't do it because you're a nice kid. Like endeavor would have had hot hands or a fleeting spark in his younger years.
But that's just my own vision of things I'm not sure it's ever been in the manga.
It could have been a bit of his quirk being pretty strong on top of that. Or better than average.
I just go with that. Like the intro said not everyone is born equal. Slar already has what is basically 2 quirks so every once in a wild the molds gets broken. It's how they came to be after all and quirks are suppose to be getting more OP.
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It's completely unrelated to the families quirk.
So Deku would have a mutant quirk(unless they think it's related to the mom telekinesis). Making Deku a little less odd to the general population. I like that.