Why are new tickets being created in open status?
When a ticket is assigned to a group with one member, that agent becomes the ticket's assignee. One of Zendesk Support's inborn ticket rules is that a ticket with an assignee moves from new status to open status. This combination of effects means that brand new tickets can be created in open status rather than new status as you may expect.
This is part of why we recommend using the Ticket | Is | Created condition rather than Status | Is | New for triggers that need to fire at the time of ticket creation.
For more information, see the article: About triggers and how they work.