Honestly, bull markets can simply be caused by the market becoming excited.
A lot of the reason why last year's bull market was so large in magnitude was indeed because of the fear of missing out factor within it, and also the fact that everyone within the market seemed to be extremely optimistic during that stretch of time. That itself does not need any causes because of the cycles that are within the market.
The bitcoin halving could also play a role, though, with the block rewards becoming scarcer. Of course, there must be new adopters to fuel this long term growth which I think it's fair to say we're seeing right now despite the bear market, with both individuals and corporations.