Same thing happens to me occasionally.
I really think Epic should reconsider their choice of not giving an option to disable starting the launcher whenever the editor is started in the binary distribution of UE4. I have to maintain my own binary build of the engine solely to disable having the launcher autostart. I’m actually considering writing a background task program that runs all the time in the system try and just kills the launcher as soon as it sees it starting. It would be easier than maintaining my own build of the engine for our team, because all of the reasons we used to need a custom build for are gone, except for the launcher being a pain. The resource usage from the launcher can mess up build/cook times, performance testing in VR, etc. So you have to keep closing it manually every once in a while or keep an eye on its CPU/memory.