@kimbo1328 I think there's a lot of balance involved.
When you're pulling your hands tend to be in front of your shoulders, not directly on your side. This means you are slightly angled back during a pullup. To prevent that you'd have to pull directly to your side which feels really awkward and difficult.
During a dip it's the opposite : the forearms remain pretty much perpendicular to the floor, meaning as the arms bend the shoulders go forward and the legs go back and you angle forward.
To transition between the two, you need at some point to move your center of gravity from the back of the hands to the front. The legs may be involved in that quite a bit because the arms already have a lot on their plate at that moment.
Now the specific reason you pike would be that you are starting to dip while your torso is still angled back, and you need to get your legs in front to tilt the COG. To prevent that you'd have to do a more explosive pullup : the higher up you go the more you lean backward, so if you only pulled for a third of the ROM and then just flew above ring level, you'd be pretty much vertical, and wouldn't need to pike to have your COG above your hands and then move it forward for the dip.
Not sure if that makes sense... I forgot to add I don't think this is related to kiping as you'll see a big pike when doing strict bar MUs too, it's possible to avoid it but there is more balance involved than momentum IMO.
