Wait what? When did all that happen? Sounds like terrible precedent.
As dinomannitro6 says, Marienburg was blockaded, bombarded, sacked and burned by Ulthuan, because a Marienburg pirate somehow got through Ulthuan's mists one time and took some stuff before fucking off back home. This could be called somewhat deserved if not for the fact that the elf in question was described (and decried by other elves) as having been excessive in his retribution and ruthlessness: he could have caught up to the pirates at sea and sunk them, but he deliberately chased them back to Marienburg to send any would-be pirates a lesson.
Later, Dieter IV (the same guy who outlawed the Colleges, as you may recall) gave Marienburg their independence in exchange for a massive bribe. Ulthuan (or at least, some of Cothique) supported this and probably gave some hidden aid long before the bribe took place.
The Emperor after Dieter decided 'fuck that' and tried to get it back, but Ulthuan (or at least just Cothique) sent an army to support Marienburg. This was the Battle of Grootscher Marsh and the Imperial army got devastated because the Colleges were still outlawed at the time, giving Ulthuan complete magical supremacy over the battlefield. Bad stuff.
And although people usually talk of Marienburg as the only part that matters, technically speaking it's the
Westerlands. And as such, they took along
two thirds of the Empire's coastline when it seceded. Neues Emskrank in Nordland (also the location of a nexus, by the way) is the
only meaningfully large settlement in the Empire's coastline - the Empire tried to establish two others but Marienburg decided to sabotage them.
If Marienburg hadn't seceded and taken those two-thirds of the coastline, the Empire would see Druchii more often and have more port towns/cities, and
definitely be able to protect itself better from such. In a very real sense, Ulthuan has contributed to this potential deal being appealing.