Demand over supply. If people buy bitcoin its demand rises and considering the limited supply the selling price of one bitcoin will rise to keep up with the profits that its traders would wish to get and hence raise the price at which they are bought at.

Bearish trends happen when bad news hit the market, like an exchange getting hacked or someone big dumping their coins or some bad statement about crypto by some big name. Such things drain the price and start a bear trend which gets corrected depending on the gravity of the news. Later on as the correction kicks in from those who were waiting for discounted price to buy at, the price starts rising again.

To answer the question in your title, I would suggest you to read the older charts and see how long bear/bull cycles last in the market.