What AI focus does is try to determine if a subject is moving towards or away from you, or staying in the same plane. Then, it selects either one shot or AI Servo.
Rather than have my camera guess (It will occasionally guess wrong), I set AI Servo unless I am capturing a image of a stationary subject at a reasonable distance or on a tripod. Even swaying 1/16 inch forward or backwards at macro distance is going to ruin a image.
Your camera detected this and used AI Servo. AI Focus is like the green button for AF setting.