The most common cause in my vicinity is headwind / tailwind.

However, there are places where the background gives subtle clues indicating an inclination (or lack thereof) at odds with gravity. This is sometimes known as a 'gravity hill' or 'magnetic mountain', or something similar. There's a long list of places in the [Wikipedia][1]

These external clues can be of varied nature - trees growing at an angle to the ground due to prevailing winds, stratified rock formations with a slight slope or long sloping hills in the distance.
  [1]: https://en.wikipedia.org/wiki/List_of_gravity_hills