I saw a discussion where the horizon was not available but you could see trees. It was assumed that because the trees were level that the horizon was also level. Whereas this is true in many cases, and certainly in the one in question, it is not always the case.
Prevailing winds, soil erosion and unhealthy trees are just some of the reasons that trees lean. I've also noticed that trees growing on a steep mountain side will often lean toward the light, particularly if the mountain blocks the sun for a significant portion of the day.
In some cases, you could end up with an image like this if you use straight tree trunks as your only guide to what is level.