i think he was actually in the process of trying to throw a check hook. thats why his arm came around that way as he was going down to the canvas.
here is what happened. khan threw a jab. canelo slipped the jab through the space that he has next to his right ear and over his right shoulder and threw a counter right hand over it in the same motion. khan saw a right hand and started the process of throwing a check hook over it instead of bringing the jab back.
when you throw a check hook over a right hand you must make sure that you have created seperation. this is what khan was supposed to try to do here.
heres michael spinks throwing a jab and creating a counter right hand and then coming over the counter right hand that he just created with a check hook. this is everything that just happened with khan and canelo but with one big difference. spinks waited to bring the jab back and created seperation. thats why khan ran into the right hand that he created with his jab and thats why spinks countered the right hand that he created with his jab.
you use your jab to do damage but you use your jab to create slipping and countering too. when you can use your jab to create the reactions that you want to create you understand the way that boxing actually works. khan is someone that has always wanted to throw an extra punch even when he probably shouldnt have. he always was in a rush to throw just one more punch. maybe that comes from amateur boxing and the amateur boxing points system. but thats not the way that boxing actually works.