A number of tickets were systematically routed from one agent. There aren't any triggers or automations responsible for the reassignments. None of my agents reassigned these tickets.
If an agent is assigned tickets in a particular group and then removed from that group, those tickets are automatically reassigned to the next agent listed in that group. In some cases, the admin removed the first agent from that group.
To break things down
Imagine there are two agents, agent A and admin B. Both agents belong to the Sales group. When admin B removed agent A from the Sales group, Zendesk automatically reassigned all of agent A's Sales group tickets to admin B.
When you remove the current assignee from a group, the ticket maintains its group status and the assignee is then shifted to the next agent of that group to keep the tickets in the group.