"You were not summoned here to grovel, Director Krennic."

Vader summoned Krennic to Mustafar to put him in his place after he threw a temper tantrum when Tarkin said he was taking control of the station. He didn't go there "to appeal to Lord Vader about ... Tarkin's attempts to supersede him."

It also never came across (at least to me) as though Vader was "someone to whom Krennic has always answered on this matter." Quite the opposite, in fact. He seems rather terrified and even Vader says Krennic is "unsettled".

I always interpreted this to mean that he wasn't used to interacting with Vader directly and was (understandably) nervous and didn't know what to expect.