Why were the Royces so steadfast in wanting to declare for Robb ? Being “near on the verge of open revolt” over the fate of Great House of a different realm seems to be indicate a very deep rooted sense of loyalty/camaraderie with the Starks, which I find surprising.

They’re kinsmen, for one thing – due to the marriage of Beron Stark and Larra Royce, all Starks today have Royce blood. And the families have kept pretty close connections in recent years – as Kyle Royce was one of Brandon Stark’s companions, it’s likely that Kyle was fostered at Barrowton with Brandon. 

Kyle’s death at the hands of Aerys II would have made House Royce one of the leading rebels in the Vale, making Bronze Yohn one of Ned’s war buddies – hence why Bronze Yohn got such a warm reception at Winterfell when he brought his son up to join the Night’s Watch.