This bitcoin halving thing happens around every 4 years which reduce the block rewards by approximately 50%. This is forseen as to help lower the supply of bitcoin into market which lead to increase of scarcity. So the problem is, will this block rewards reduction actually lead to a price increase due to scarcity or if the market has already priced it in?
This has been the trend since the inception of Bitcoin and that has made it a reason most persons now believe it's most definitely going to follow that same pattern, in economics, there's a law that states that when things become scarce the demand increases which causes the price of such to inflate, so applying that to Bitcoin halving, what's expected is the increase in price there after the halving but then this information isn't a financial advice.
This has worked in the past years and many others have benefited from this so it has become such that people have trusted to probably happen in such way again since it's looking like it now the norm after it has happened for several years following the same pattern.