The Vikings had managed to drive back the army but Tyr knew that they would return, they had to leave the area or they wouldn't survive the second wave.
Bjorn was given a mission by his father and he needed to find his brother right now otherwise they would lose their father.
Bjorn and Salgard left the area with Grjötgard still engaged in battle, Bjorn momentarily thought about leaving him because this would allow them to escape.
But this wasn't who he was, this was something his younger brother would rather do.
Bjorn rushed to the Grjötgard but Salgard wasn't bothered and instead, chose to climb back to the wall.
Salgard knew his brother wouldn't fall regardless of who his opponent was, Grjötgard was far too strong and if his opponent was alive, it was only because he wanted him to.
If Grjötgard killed his enemy, then even stronger ones would come and his threat level would be increased.