The passing driver could be slowing down for many reasons, some of which might be:
-The car doesn't have or isn't using cruise control, and the driver doesn't realize he/she is slowing down, doing it unintentionally.
-The passing car isn't actually slowing down, and the car in front of you is speeding up. The car might not have cruise control, or the driver might not be using it, realizing how slow he/she was driving and having sped up.
-The passing driver sees brake lights at the top of the next hill, and let off on the throttle.
-The passing driver sees a possible police car ahead. It might be in an oncoming lane, on the median, on the shoulder, or traveling in the same direction.
Whatever it is, I don't think pondering the cause in any particular scenario is a productive use of your brain power. It's best to accept this phenomenon is one of the things you will never change.