Yeah, but Ragnar looked particularly agitated while talking to that head. Even Bjorn noticed it and asked "are you alright". Something serious must have transpired between Ragnar and that severed head.

That severed head had an uncanny resemblance to Floki. There's a good chance Floki was mumbling or moaning about Aslaug while having that vision, causing Ragnar to kill him in a fit of rage. He was high on Yidu's "medicine" after all.