Ragnar didn't know the fate that awaited his son but if it was anyone that could survive this, then it was Tyr but fate was an unknown variable.
Fate was the one thing that decided what kind of life that man lived but not everyone believed that their fate couldn't be changed.
Osberht was gaining but the Vikings were closer to the ships as well. However, they needed someone to confront the enemy or they may not make it.
Grjötgard realized this and stopped his horse, Salgard didn't look concerned by his brother's act because there was no way he was going to let him have all the fun.
Salgard stopped as well with Ragnar giving Grjötgard a thankful nod but Grjötgard saw that Ragnar was willing to sacrifice his son.
That was a man he could respect and thus, he was willing to confront the army before him.
He knew who the leader was and thus, Osberht became his target.