Here's an idea:
Tesseractia
Earth & Space
The world is four-dimensional.
The very fabric of space folds inwards upon itself in Tesseractia, creating a 4th-dimensional axis, in addition to length, width, and height. This new axis, referred to simply as 'depth', is difficult for rational beings to navigate. (However, more intuitive beings, such as insects and lower-order animals, seem to have no trouble maneuvering across depth.) A flight of stairs may appear to go upwards, but at depth, a very similar flight of stairs goes downward. The human mind, unable to grasp the different sets of stairs, intuitively shifts to the depth that is easiest for them, in this case: the one going downwards. Likewise, a river system may have many branches and loops when viewed from a complete 4-dimensional perspective. However, the human mind cannot grasp this concept, and instinctively finds itself maneuvering into whichever branch flows in the direction they intend to go.
This effect does not extend more than a mile or so into the atmosphere. For airships that descend towards the surface, transitioning from 3D space to 4D space offers no problems: there's simply more space to fly in. However, to depart Tesseractia, the airship must ascend out of 4D space into 3D space, which can be a very rough transition, and requires expert piloting skills to hold the ship together.
Anyway, just something off the cuff. Combine that with other Ob planets, and have their combined effect "net out" to the Downward proposal. But if the PCs want to tease apart a component, here's an interesting bit.