Technically nothing stops them, but in its current form, if any of the sub-tribe members stamp the original leader, member set is back at the original tribe, but with a reorganized hierarchy. To start a new tribe, - interference from the old tribe should be minimized - everyone down the hierarchy should acknowledge the new leader and his rules. I'm still thinking about ways to make real forks happen technically. If the new leader says "I accept old tribe's stamps up until the fork timestamp, and here is the new context that you have to use in your stamps", then it's possible I guess.