veer
To suddenly change direction or course from your path.
To veer means to suddenly change direction. When a car veers off the road, it swerves away from where it was heading. When a bicycle veers to the left, it turns sharply instead of going straight.
You can veer physically, like when you're running down the sidewalk and suddenly veer around a puddle. But you can also veer in how you're thinking or talking. If a class discussion about ancient Egypt suddenly veers into talking about pyramids on Mars, it has shifted away from the original topic. When someone's telling a story and it keeps veering off course, they're getting distracted and losing the main thread.
The word carries a sense of suddenness and often suggests something unplanned. A ship sailing steadily south that veers west because of strong winds didn't choose that direction. A conversation that veers from serious to silly probably changed without anyone meaning to guide it that way. When you notice something veering, you're watching it move away from its expected or intended path.