Dawnguard updates the MHDT in the cells around the castle to ensure that dragons won't try to fly through it, but later DLCs mistakenly revert the height data back to pre-dawnguard Vanilla data, and USSEP was using the final master rather than Dawnguard. This means that if a dragon spawns there, it will try to fly right through the walls.
I just recalculated the MHDT in those cells entirely to ensure we got the most accurate. (Looks like bethesda also forgot to update it for one of the cells even in Dawnguard)

Attached Files:
volk.esp