Bitcoin price is purely determined by a combination supply and demand, and no one really understands them. Is the price growing because supply is decreasing or because demand is rising? We can't answer even such simple question, because we don't have the strong data.

One of my theories is that these cycles are a kind of self-fulfilling prophecy. People look at the past pattern, assume that it will repeat and act accordingly. If the price rises before the halvening, speculators think it's a premature rally and dump their coins, causing a correction.