What is happening:
Each time you jump when you are walking up a steep hill (not extreme, just steep), you jump, but then you slide along the ground with little or no control, until it decides you are on the ground again.
This glitch occurs reliably on hills with a large enough grade, although it occasionally happens on lesser gradations.
Very annoying as an earth pony in Cloudopolis, where a lack of control means that you fall off.
Will not occur on near-cliffs. (although being able to climb those is another bug)