The same law which makes gun recoil happen. If you fire a pistol in vacuum you would still get the same recoil or even stronger. The rocket engine fires a lot of gas molecules instead of bullets at much higher velocity than a bullet, which gives it the constant push/recoil
I remember when Falkon 9 was doing its first landings, the whole YouTube comments section was filled with flat earthers claiming it’s a CGI. Now you can take a car and go watch landings in person, I wonder where all those people went.