The causes for beginner's luck are unknown. It is speculated, however, that beginner's luck arises from a disconnect between the player and the pressure of the game. A novice player is inexperienced and consequently is not expected to do well. This means that there is no pressure on the player to excel; this lack of pressure allows the player to concentrate more than a pressured veteran player. This goes against the Rosenthal effect which states that students who are expected to perform better usually perform better.
Beginner's luck is thought to end once a player gets involved with a game. Once the "innocent" psychological mindset is replaced by one that is concerned with the nuances of the game, concentration goes out the window and skill level decreases.
Another explanation begins by noting that the acquisition of a new skill imposes limitations on the number of actions available to an agent. In the early stages of this process, an almost unlimited number of actions are possible. Though almost all of these are ineffectual, the probability of unusually effective actions manifesting by chance is still greater than when one has attained a moderate degree of skill, since, as one’s ability improves, the scope of possible actions becomes both more lawful and more limited, subtending freakish deviations from the mean both directions. Due to the availability heuristic, these runs of flukish proficiency will stand out against the base rate of general ineptitude.
A final explanation is statistical. Suppose that 100 beginners play a game for the first time, and half win by random chance. The half that won is more likely to take an interest in the game and become experts, while the half that lose is more likely to lose interest and never play again. Thus, is any game, the "experts" will believe in beginner's luck, simply because they disproportionately experienced it themselves.