Stampede at the Gate
I first noticed this years ago while travelling in the US, and while there was still assigned seating. The only thing I could think of was it was an attempt to secure a place for the vast array of hand luggage that was going onboard. I don't know if it's still that way over there, but perhaps the idea spread beyond US borders. The only other reason I can think of is habit borne of LCC boarding practices. I agree it has made boarding genuinely unpleasant so much of the time.