From what I understand it is a popular game engine many developers have used in their games, and are still developing games with. This happened under a regular licensing agreement, which usually means a fixed sum or something tied to sales.
Now unity had the benighted idea to unilaterally add an additional charge to this, billing the developers for each and every installation of their game, which is not only incredibly greedy but also short sighted and ignorant of many common cases that would inflate those numbers (reinstallation for any reason being one of them).
Naturally developers affected by this are quite upset and are considering possible solutions to this self inflicted hostage situation.