I've had this happen to me on occasion. It's a good thing, I think.
This happens to me when I find myself deeply immersed in the story.
When I first began writing fiction, I'd take my characters and "force them" to do thinks, like they were marionettes. It never worked.
As I grew in skill, I then learned to create situations, place my characters into the plot, and sit back and see what happens. In effect, I was taking dictation, just being an observer.
Naturally this is all happening inside the brain. What a complex joy we all have between our ears! I'd actually separate the story events from my immediate consciousness and detach myself.
This aids the creativity, as you've now learned.
My suggestion -- SAVE your current story progress in case you need to go back. Don't delete any files. Then let it flow, write your tail off, and see what comes out the other side. It will most likely be far better than any "forced" plotline.
The end result will probably be a blend, because the "forced plot" does have some possible advantages -- more coherence, more detailed history line, etc. So you can take the best elements from both threads, the carefully planned thread and the impusive one, and use the best from each.
When I'm making big changes in a story line, or when I "feel it coming on", I save successive versions. For example, my most recent novel "Full Circle" will have maybe a dozen incomplete files in a computer folder: "full01.doc", full02.doc" etc. I simply "Save As" with an incremented file name, to keep all the various interim versions.
Keep us posted on the results!