It's possible to have a fork where both sides of the fork remain compatible. A hostile fork will lose this property by one or both sides of the fork causing divergence on purpose (generally the incumbent side) to make it difficult for the other side (generally the parasitic side). This is certainly a real phenomenon, regardless of whether the software is free or proprietary.

> What? Matrix is Free Software. There is no such thing as "hostile fork".

Of course there is.