Kaido betraying Orochi was something that was always up in the air, both of them were just using each other. Orochi was using Kaido as both a bodyguard and also as someone to put fear into the Wano citizens, meanwhile Kaido was using Orochi as a proxy leader while he was building up his crew and making weapons in Wano.
Now with having a good crew size, enough weapons, and the alliance with Big Mom, Kaido doesn't need Orochi anymore.