Comment by Jopp Hyung on Exclusive Contract is Difficult, Your Grace - Chapter 18

So the dragon most likely sees the mc as a threat, since dragons in this case are much like dogs and can sense fear, seeing that fear as a sign of aggression and leads them to attacking the strongest one or the most intimidating.
Most likely in attempts to protect Lady D since she looks like a squirrel and is most likely seen as the least threateningthing in the world.