Grjötgard confronter Wilfrid but there was a communication barrier so it wasn't to talk but to show that they weren't intimidated.
There was no doubt that his size was intimidating and it was impressive that a horse could carry someone that big to begin with.
The Christians with Wilfrid were different, they had seen the harsher side of the battle and hence weren't intimidated.
Grjötgard unmounted his horse and stood right in front of these people but they didn't react with hostility.
This told the General that they were level-headed and with this single act, he could discern that they would be a lot harder to rile up.
Grjötgard unmounted his horse, the Vikings within the walls were surprised that he would even attempt this.
They might have the upper hand but there was no telling what this single act might result to.