Been there, done that.

Why does it happen?

In my experience, a split due to creative differences usually happens because different people have a different idea of what the project goal actually is, but nobody is aware of that. As soon as a contributor realizes that someone else's vision is different from theirs, arguments and power struggle will start, which can quickly become personal and tear a project apart when not moderated properly.

How to prevent it?

The best way to prevent this situation early on is to communicate your creative vision of the project early, clearly and often. Every project should have some kind of official document which outlines the goals. Make sure every contributor knows and understands it, so everyone is on the same page and nobody gets any misconceptions about where you are heading. Make clear that anyone who wants the project to go somewhere else, should fork from the start and not get involved in the mainline in the first place.

Should there be any disagreements about aspects of the project direction which were not set in stone beforehand, it is important to make a binding decision before things turn ugly. Having a clear hierarchy or well-defined decision making process is fundamental here. Without a binding process to make a decision - whether autocratic or democratic - people have no other choice but to "reach consensus" by either talking down the opposition through countless hours of filibusting in your communication channels (time they could rather spend working) or driving them out of the project through bullying and intrigue (a lose-lose situation for everyone involved).

Unfortunately, when the conflict is already under way, it is likely too late to establish a proper decision-making process. Such a process only works when everyone supports it. But when you try to establish it now, everyone will perceive it in the context of the current conflict and their support for it will depend on whether this process would decide the current matter in their favor or not. Opening up this new battlefield now will likely deepen the trench instead of bridging it.

The split still happened. How to deal with it?

When the project is under a copyleft license (or when under a permissive license the other group is committed to keeping), you can still merge any of their commits into your codebase or vice versa, so the manpower is not completely lost to your project. But a split is still a considerable blow to the project because organisation structures and infrastructure need to be duplicated, coordination between the forks is impaired and their commits need to be carefully reviewed for relevance and merge conflicts.