watch tons of street fights (real aggression, not two guys fighting for the camera), and read articles on this subject, as I'm primarily into self-defense over combat
sports as far as my training and instruction goes. From what I've seen, about 90% of real fights hit the ground from a clinch.
The reasons being:
A) The average joe fights squared up to their opponent and throws only hooks. Hooks force both guys to close distance, which eventually turns into a clinch at close range.
B) Its the natural reaction for an untrained person who gets hit in the head, to reach out and grab onto something to steady themselves. That "something" tends to be the person in front of them thats hitting them, and we once again have a clinch.
C) A lot of street fights start with at least one person being enraged, and people in an enraged state, or "condition black" tend to rush their opponent, either with a punch or a tackle. The punch, unless it hits the guy flush and drops him like a sack of potatos, is really going to end up as an entry to a clinch.
D) Alcohol is involved in about three quarters of street fights, so theres the natural unsteadiness that goes along with too much to drink, that will send people toppling into each other and onto the ground.
The top three senarios end up in a clinch, where several things happen, which generally leads to the fight on the ground.
A) Being as neither guy is trained in a clinch, it generally turns into a greco match with two guys locked up and pushing each other into walls, people, cars etc, which trying to hockey fight with close range punches. With neither guy knowing how to move in a clinch, people tend to trip themselves on curbs, chairs, or on each others feet.
B) Some guys naturally go for a sort of twisting takedown, which leads to a ground fight, as the guy getting thrown is naturally going to hang on for dear life.
The only times of seen street fights NOT go the ground are from one or two punch KO's, from a huge size disperity, where the little guy can't physically effect the larger mans mass by grabbing or tackling him, or with skill disperities, where someone just beats the living shit out of the person their fighting.
If anyone is looking to see real street fight footage, the best thing out there are shows like Cops, real tv, Max Ex, and the news. To a lesser extent, "reality shows" like Springer, Cheaters, etc while stupid, are packed with real life fighting between two people, and gives you an idea about how untrained people tend to conduct themselves in a fight.