When we talk about Bitcoin volatility, I always consider the Bitcoin total supply which just limited to $21 million, a great drive for it's market sentiments in my opinion.
Now, besides the limited supply of Bitcoin, there are many reasons why Bitcoin is volatile. these include, Demand, scarcity, Speculation, FOMO, Fears etc.
The more the increase in users, the more the demand for Bitcoin. In facts, the continue demand for Bitcoin bring about scarcity while big holders(whales)will always try to control the market through speculation. when the start market going down, then comes in Fear, also when Bitcoin start pumping there may be Fear Of Missing Out.(FOMO )