You are at least half right. The new crews are not out of nowhere though - the builders know who the good foremen are, so when those foremen go on their own they will take them. Builders don't just take bids from anyone, they call the companies they always use to bid on a job. The builders often keep the same crews busy year round: there is no bidding, just at the end of one job they tell the crew where the next job is. When a builder has more work than crews they contact their current sub contractors and ask if any want to expand.