Reviewing the "Triggers and Orders of Execution" document leads me to believe that only workflow rule field updates cause Update triggers to fire if field updates occurred. I'm seeing an instance where an assignment rule is changing the owner of Case record being inserted and it is causing Update triggers to fire. No workflow rule field updates are occurring. I see in the debug log that workflow rules, assignment rules, and auto responses are all prepended with WF as in "WF_CRITERIA_BEGIN". Does this mean that workflow rules and assignment rules are considered a form of "workflow field updates"?
Anything that results in a change to the object will result in causing an after update trigger to fire. So if an assignment rule affected the object, the after update will fire an additional time.
Its possible there could be some kind of related workflow associated with the assignment rule, particularly since you're seeing "WF_CRITERIA_BEGIN" in the same section of the debug log.