I did have a feeling that it may be something to do with drastic elevation changes over mountains. Although I did try sending a darter out over the ocean and after a couple km it stopped.
Side note. It would be VERY awesome if you could set a fixed altitude. So instead of the UAV having to climb every time it sees a mountain, it would just cruise over it(as long as you set a high enough altitude).
As well if you could enter in the altitude rather then choosing from the few presets it has.