English gardens are often designed to appear wild. A garden path wanders, showing you only a small amount of what is there at any moment. Thus a "garden path sentence" is one that wanders from one subject to another. You are meant to enjoy the experience of walking through it, not the beauty of the master plan of angles and viewpoints (as compared to most Continental gardens, like Versailles or Schoenbrunn). There are exceptions on both sides, of course.

But the point is that "garden path sentence[s]" are more random, more free and loose.