The topic title is simplified a bit... But that's the most noticeable effect, anyway.
First, in case you wanna try this yourself, it might be handy to know that the X and C keys control the camera pitch - you don't have to use the mouse.
What's happening here is the game is taking the direction the camera is pointing, and trying to move Pepper in that direction by a certain amount. This amount actually changes depending on the FPS, such that Pepper's speed won't increase/decrease if the game's framerate increases/decreases... But let's just say it's trying to move Pepper 1 unit in the direction the camera is facing.
If the ground is flat and the camera is facing straight forward, this will just result in Pepper moving forward by 1 unit. But let's say the camera is facing mostly downwards. This also means the game wants to move Pepper mostly downwards - but there's ground in the way. So what happens?
Welp... This. He doesn't move as far/runs more slowly.
Of course, the ground in LI2 isn't perfectly flat, so the same happens when Pepper starts climbing slopes. And as you can see in the second video, some slope + camera angle combos are enough to not only make Pepper slow down or stop, but start going backwards while still walking forwards.
Needless to say, this isn't really the best way to calculate these things.