I could only piece two possible situations from this.

1. Kabuto is actually betraying Orochimaru and/or conspired with Tsunade somehow to kill him.

2. Tsunade was actually planning on killing Orochimaru and Kabuto stopped it before it happened.

Either case there are a lot of confounding variables. One being Tsunade didn't bring two sacrifices to revive her lover and little brother.

But a lot of variables like that usually means a very good fight.