As for many massive and durable wars, the problem lies in a dual-cause. This cause is the strategic incentive to war for many of the kingdoms engaged in the process.
First part of the dual-cause, "Thucydide's trap": This is a phenomenon in an area with different entities of common culture, that have no "ideological" reasons to fight each other (example of the contrary: the Crusades). But one of them becomes stronger, and the others don't want someone to be so uniquely strong that it could become the King. So some sort of coalition could raise against the strongest entity,and this results in a long period of war during which, every time an entity stands out of the crowd, it is attacked and put back by all the others.
Second part of the dual-cause, strategic opportunities: On the other hand, when you feel you're strong at a time but the others are going to catch up, your entity feels like it should attack and secure its positions. This could lead your entity to partial or full-target attacks, that creates a war.
Other reasons had come in from time to time:
- Alliances: when you had good relations with an other kingdom that is attacked, the First Part gets more important as you don't want to be isolated by having all your allies defeated
- Starvation, trade issues: sometimes you need something and your neighbour don't want to give it to you...
- Plunder, raids: sometimes one of the soldiers or citizen under your responsibility acts without order and you need to catch up with him, otherwise your population will see you as a traitor and/or a weak man