Why does BTC increase so much 12-18 months hafter halving

Just wondering why this seems to happen. Most of the hype seems to be around the halving but then 12-18 months later the price rockets and I don't understand why. To me that seems to be when the hype has died down. If it's supply and demand then this time should look much different because of the ETF demand correct? I know a lot has to do with psychology but will those who invest in the ETF buy into the BTC cycle theory? Just curious if anyone has any insight.