That is one thing that worries me when a house needs the building envelop broken. The roof or back wall is ripped open and the contractor sees an unforeseen problem and needs a ton more money. He may have noticed or suspected the problem during the quote stage but held it as an ace up his sleeve.‘Eff you pay me, or I won’t reach substantial completion’.
There’s a LOT of things that go into it, and there’s blame to go all around.
Agree to pay up within 48 hours or the guys go to a new job and won't be available again for months. It may rain and your house is unlivable.
If you bring in another contractor the original one can place liens for loss of profit etc. This discourages the second contractor from taking on the job.
It happens to government jobs as well. I was involved with one where the lowest bidder became the most expensive and slowest. It was basically blackmail by inconvenience.
Performance bonds seem to be a good idea but if I was asked to guarantee a result I would probably triple my price.