A possible cause is related to how when our brains learn new skills, especially complex ones, it's through shaping (step-by-step/successive reinforcement). With repetition and practice, we can recall this action with less and less needs for reinforcement/reward/conscious effort.
A related theory is the concept of long-term memory having sections of explicit (conscious, intentional recollection) and implicit (unconscious, procedural, muscle) memory. Procedural memory lets us perform some actions (such as writing or riding a bike) even if we are not consciously thinking about it. In one experiment two groups of people, one composed of amnesic patients with heavily impaired long-term memory, and the other composed by healthy subjects, were asked several times to solve a Tower of Hanoi puzzle (a complex problem-solving game that requires thirty-one steps to complete). The first group showed the same improvements over time as the second group, even if some participants claimed that they didn't even remember having seen the puzzle before. These findings strongly suggest that procedural memory is completely independent from declarative memory.
In situations with a high amount of pressure, such as a freethrow at a basketball game with four seconds left on the clock, we are tempted to go back to the explicit step-by-step process again, basically ending up interfering with the learned skill, and suddenly that move that you've done thousands of times and could probably do in your sleep becomes awkward to perform. This is one of the biggest causes of Performance Anxiety and why some athletes have a tendency to "choke" under pressure. In Real Life, it could be argued that, for instance, learning to play a musical instrument is mostly a process of overcoming one's Centipede's Dilemma. Or swimming when you don't really know how.