[The Might]

I don't know if this explanation is too simple, but perhaps just that they all lived in Dunland, at least for some time?

Dunlendings - of course lived in Dunland
Stoors - crossed the Redhorn Pass and most of them settled south in Dunland
Dwarves of Erebor - were led there by Thráin after they were forced to flee

Perhaps one could add to this that they actually wanted other lands than Dunland.

Dunlendings - the rich lands of Rohan
Stoors - eventually made their way to the Shire or back to the Gladden Fields
Dwarves of Erebor - went to the northern Blue Mountains and later Erebor

EDIT: by the way I thought about the lembas episode with Frodo and Gollum but forgot that Beleg had done the same with Túrin.