That's one of downsides of Southwest's point to point system. With a hub and spoke system (at the big 3 legacy airlines), many planes are much more likely to spend the day going back forth between one hub and one or two spokes. Only if that hub or those spokes are affected (by weather, etc) can it cascade for later flights on that plane.
But with Southwest running their point-to-point-to-point-to-point planes all around the country 'at random", weather/etc issues in one part of the country can "randomly" appear as delays later in the day in other unrelated parts of the country. If you don't know the route the plane took through the day up to then, you can't figure out whether it came from an area affected by weather/etc or not.