Main reasons are thru buy and sell but what we have to point here are main reasons why people buy and sell.

Let's put it in simple terms like "someone is buying" and "someone is selling". Yes, it's about market behaviour wherein bitcoin price was moving because of buy and sell. Now for how much it will rise or fall will depend on how much volume it will made.

Different reasons also are associated whether people will act to buy or sell. Some are due to speculations, some are due to riding the current trend, some will just do normal buying etc. , there are really lots.

Soon as you continue to progress in bitcoin world, you will understand more how bitcoin price behaves without knowing technically the "specific" reason why it was moving drastically.