The auto-assignment feature will assign the tickets to the agents as per the order in which they have been added to the group. For example, if agents C, A, and B are added to a group in that order and if they are all online to accept tickets, the tickets will also be assigned in the same order. 

Therefore, if the tickets have to be assigned in alphabetical order, please manually rearrange them accordingly in Admin > Team > Groups > click on edit to achieve this.