Stamford Bridge was basically why did Harold march North -
I put
The Hardrada invasion, and explained his claim - he was willing to follow it up with invasion so Harold had to fight to protect the country and crown
The original plan of defence had failed because of the defeat of the northern army at Gate Fulford, so Harold had to fight at Stamford lest he be surrounded when William arrived
And Hardrada could expect support from the people in the Orkneys given they had Scandinavian links from an earlier settlement, so Harold had to fight to prevent a Northern uprising, especially given that he had lost troops at Fulford.