every 10 minutes a new block is mined and added to the chain. When that happens 12.5 bitcoin are released to the miners. That means 1800 new bitcoin are minted every day.
Sometime in the middle of May the block rewards will halve. That means means that instead of 12.5 it'll be 6.25 and the daily amount of btc minted will drop to 900.
Miners typically sell because they have to pay for their electricity. So, for easy math we'll call btc 10k. When there's 1800 a day there's 18M USD that gets sold on the market from miners. When there's only 900 a day the weakest miners get rekt, a much smaller amount of bitcoin is produced, there's only 9M USD a day of selling generated from teh miners, but presumably there's still 18M worth of demand. So, price starts climbing, or at least that's the bullish argument.