There always seem to be openings this late due to people getting cold feet. Also there are those who have legitimate reasons that have to do with family, school or job/internship opportunities. It's different with every scenario, but it's not impossible to fill their spots. I never get too worried about it.
A little side note, Phantom had huge issues keeping tuba players throughout move-ins and still ended up having a great season with a fairly decent section. No small feat!