No, they are casting spells that increase the weekly growth of Elves, which significantly increases the enemy's troops (they tend to move along roads and collect them).

It seemed to me that it's not weekly growth, but a doubling of the Elven neutral troops + the appearance of new ones, because in the first week, packs of flies with a number of less than a hundred are scattered everywhere, and by the 4th week, these packs grow to 300, and it becomes difficult to move without losses. Also, the squads of dancers and archers increase. Perhaps this problem arose because I am playing on version 3.1 with a mod to unlock all campaigns?