Really this.
Before Legion she was basically just living her life taking care of her people. Sure she made a deal with the Jailer to come back with her Val’kyr but up until the point Vol’jin is stabbed she is really just a sleeper cell. Doing her own thing, thinking she got the better of the deal with the Jailer. Then she gets activated and told she has to be the new Warchief because the Jailer has arranged it so that Vol’jin names her as his successor.
I don’t know about you but if I’m just sitting back coasting through my job for a decade and all of a sudden my boss tells me I have to be team leader and follow through with the biggest project ever, I’m not gonna be happy about it. But a year down the line when it looks like I’m gonna pull it off, I might just have changed my mind about the situation.