It happens to plotters and pantsers alike (although, admittedly, more to pantsers): your book goes off the rails into a new direction, and you don't know where you're going or what's going to happen when you get there. Your main character had done it again and run your novel off your envisioned track. While you might find a bit of serendipity here if a great plot twist reveals itself, it's more likely going to end up with muddled chapters and a complete lack of direction. Sooner or later it's going to bog down with your story going nowhere, fast.
Infinity's Blog for Authors and Writers
by Sherrie Wilkolaski