I kind of have a theory why wu caused the merge (but it's more of a fanfic idea so it probably doesn't make sense)
But basically, Wu caused the merge to protect Lloyd
When the first merge would have happened, the first spinjitzu master, who was probably a conduit, was there to prevent it. So I think that merges have something to do with conduits.
The first spinjitzu master might have had a vision about a second merge and another conduit. But he also knew that the conduit would come from his bloodline. (Doc/someone on twt said something about a green ninja related prophecy we don't know anything about yet)
The first spinjitzu master passed the knowledge of that to Wu, but he wasn't specific enough.
Wu figured out that Lloyd would be the next conduit on his own. (He comes from the fsm's bloodline and is already a powerful green ninja)
And because wu didn't know where the dragon cores were, the only other way to prevent the merge would have been to get rid of Lloyd, which he just couldn't do, and didn't want to. The reason the merge happened too early was because Lloyd's age had been messed with (tomorrow's tea)
But that's basically it (hopefully I'm not talking complete nonsense, but this is more of a fic idea sooo)