Wild theory based on another theory that the Dravanians are on Hydaelyn's side...
Tiamat sees that Midgy is with us, and wants to act as a mediator between the scions/ishgard and Nidhogg before the ascians can complete their plans. His brother has gone mad with rage for too long and lost sight of Midgardsormr's pact with Hydaelyn, not realizing that the dragonsong has now escalated her weakness even further.