ok, i'm throwing ideas at the wall here, tell me if i should stop:

there's rumors of an old crone going out into the forest at dusk. people think she's pactising with chaos to curse the baron's family. in fact, she's misguidedly praying at a beastman altar to ensure a good harvest. upon inspection, she's just a very misguided old lady who thinks she's a magic user (but isn't).
said prayers don't fall on deaf ears however, and a beastman shaman is indeed gathering a warband to raid the village. he's sent envoys throughout the forest to gather even more.
the constant braying at night is interpreted by less scrupulous folks as a sign that sigmar has abandonned them, and so go all quisling on the empire, and start praying to the beastmen (and so the chaos gods) for safe passage through the night. magic powers in the population start to manifest, including a little girl that sets feathers on fire with her mind.
a travelling witch hunter hears of this and comes to check out the village. in reality he's a double agent and searches the lands for new recruits in the name of tzeentch, the god of magic. everyone he rounds up is taken outside the village. everyone he burns is someone who's seen through his ruse.
magic phenomena happen at night, scaring the populace into searching for even more witches. one night, an angry mob gets attacked by a herd of beastmen.

maybe the siege engine could be used to take out a herd? i don't know.