The generally accepted belief is that these young and militarily inexperienced brother Earls (Edwin of Mercia; Morcar of Northumbria) had only their own independent Northumbrian and Mercian interests at heart, and so betrayed not only King Harold by failing to show up at Santlache on October 14th, but also the nominal young king Edgar afterwards by fleeing London, intially.

But is it as straight-forward as this?

A first cousin of theirs, Abbot Leofric of Peterborough, was actually present at the battle at Santlache (Hastings), so their family's involvement in Harold's cause was taken seriously, despite the traditional antagonism between the Godwinsons and Edwin & Morcar- and their renegade father, Earl Aelfgar, before them.
They also had a vested interest in seeing Harold succeed- their own sister Ealdgyth, the new and official queen, was pregnant with their king's child!

In all fairness, they had resisted Tostig's raids in Lindsey, forcing him to abandon his attacks and seek refuge in Scotland with King Malcolm III, his 'sworn brother'. This could be seen to be more because of their hatred/fear of the ousted Tostig's vengeful wrath than of loyalty to Harold.
Then they blocked the route to York from the Norse invasion army at Fulford Gate under King Harald Hardraada/Tostig, but lost that brutal pitched battle, fled the field and were in no position to fight on with such devastated forces.

Whether they were at Stamford Bridge or even Santlache is a moot point, it is of course very possible that they were, with remnants of their bodyguard, under peer pressure or loyalty. Certainly someone with the authority and notoriety led fresh troops and panicking men at the 'last stand' at the Malfosse.

They did not submit to William immediately after Oct 14th, but rallied around Edgar in London, briefly. It was only after the witan and churchmen chose to submit to the duke that the young Earls joined them at Berkhamstead.

In a few short years, during which they don't appear to have collaborated with William's new regime, they escaped from his gaols and raise revolt against him, with Hereward at Ely in 1070-71.

So, were they really traitors?