Hello David
Even though a Cisco switch port configured with switchport mode dynamic auto is passive in terms of initiating a trunk negotiation, it still sends out DTP frames. This behavior is part of the DTP’s design. If you take a look at the capture, you’ll see that the DTP messages being sent tell the other end of the mode the local switch is set to. It includes information on the administrative and operational mode of the local port. This information simply tells the switchport at the other end that “I am DTP enabled, and this is my status.” That specific DTP message won’t actually change anything on the other end, since the DTP configuration operates passively, but it is informational in nature.
I hope this has been helpful!
Laz