We all know Ali was thick headed but standing flat footed with Joe Frazier not once but twice was not the smartest thing. These fights were all about just proving a point it seemed.
Like when Ken Buchanan went down in the 1st vs Duran from what looked like a blow to Buchanan's glove...he was off balance then and went down.
You can clearly see their feet hook together before the camera goes too high to see. Frazier's left foot catches and drags Ali's left foot outward and this is reflected in the way Ali falls. He falls to his right onto a bent right knee because his left leg has been pulled sideways causing his fall to go to the right.
Solid punch and Ali was probably buzzed by it as he had been getting tagged all night by that hook. With that said I believe it was a slip.