I think these three things influence an army.
1) Unit maintenance cost
2) Sharp rise in CS
3) Gold income
1. Aurvandil has income of 3-4k gold per week. They have 44 nobles in their realm so they might be able to increase the tax rate of their city if they have many nobles working for the city.
2. Aurvandil attacked Tower Fatmilak with around 2280 men. That means each noble leads an average of 51~52 men. Having over 50 men will probably cost at least 8 gold a day, 56 gold per week. Having 44 nobles leading 50 men each will cost over 2400 gold. That is over half of the total income.
3. Aurvandil managed to increase their CS by 12000 within 14 days. (Started around 16000 CS on Jan 11th to 28000 CS on Jan 25th)
You third one is the most telling in my opinion. They may be working with an army they can't afford long term. Given the gold reserves and the RCs that is easily possible. In terms of cost to pay units, 50 of my average stats archers cost 70 gold per week.
