Uncontrollable things can't be managed or influenced. An uncontrollable three-year-old can be a handful, while a politician with an uncontrollable desire for power can be dangerous.
If you fly off the handle at the smallest irritation, snapping angrily at the people around you, you have an uncontrollable temper. And when you and your best friend burst into giggles in the middle of chemistry class, it's uncontrollable laughter that's going to get you in trouble. When something is controllable, it can be restricted or restrained. Add the "not" prefix un-, and you've got something uncontrollable.