Okay I'm missing something. When did the serbs join the anti-byzantine coalition?
While the Romans were focused on the Timurids I supposed they were convinced by the Venetians that in joining their alliance they could prey upon the romans in a moment of weakness when they were over-stretched and lost significant manpower. Essentialy they took, along the bulgarians, a huge gamble, now they will have to win at all costs (but it's not they didn't know it).