Why do tickets automatically assign to one agent in Zendesk Support?
If your account has more than one agent but there is only one agent in a group that can be assigned tickets, all tickets assigned to that group are automatically assigned to that one agent.
If none of the options above apply to your situation, then a business rule is auto-assigning the ticket to a specific group. To find out if a business rule acts on the group of the ticket, see Viewing all events of a ticket.
For other questions regarding assignment issues, see the articles below.