There is lag in the animation when you use special attacks, it could be different as per the champion you use but when you use a special attack, like special 1 of an champ , till start to finish the attack, at the end there is a animation who takes place before you get the control back of your champ. This is the spot or exploit that AI uses to attack on your champ , you are venerable at this point cause you don’t have control on your champ as your champ is finishing the special attack animation.
The below video will explain the situation, when SW finishing her special attack animation and she got hit by dormammu right when she about to finish the animation and you are getting ready to get control back of your champ.
https://youtu.be/5V-saePVat4