While I understand the need for sometimes assigning seats, when a professor is asking a student to change seats, the student is owed an explanation. These are adults, and it isn’t the military, and “because I said so” is not an appropriate response by a professor to an adult. This is especially true in this case, in which the student was not being asked to return to his assigned seat, but to move from one unassigned seat to another.
The professor was focused on his ego, rather than on the best way to run his class.