More than one, it depends on the team, and the situation.
Obviously, if you're a **** team leader and you don't make the effort to make sure your team members are happy with their work and are progressing well then your organisation is going to fall faster than the bosses from Serious Sam.
If your team members refuse to get along with each other, you're buggered to high hell.
The rest of the reasons are all applicable, but not to the same extent.