Alexander was left a bit astounded when he heard that 10,000 men were able to land right inside the city with no one suspecting a thing.
10,000 men did not simply appear out of nowhere.
The number of ships carrying them should have been huge, and visible for miles to everyone.
So the only way that Alexander could think how they were able to land so many men were either through gross incompetence, willful ignorance, or because they were sanctioned to.
And to know which of these it was, he needed to know when they landed.
Because if the 10,000 men landed before the plague and both Lord Margraves IV and Lady Miranda's son Marcus were alive, i.e. the time things in the family should have been stable, then it must have been under the former head's permission, resolving both Lady Miranda and Lord Cassius of any guilt.