Since this thread stopped after the spring migration I figured the fall migration should also come into discussion.
Firstly, geese are birds which prefer to be in flocks. Ideally of their own kind. We get thousands of pure flocks of Canada & Snow Geese during both spring & fall migration.
We also have numerous resident flocks of Canadas. I have never seen a Snow, Richardson's, or Ross' Goose with them during the breeding season, or through the summer months. I have seen domestic geese associating with them post breeding, and I guess it's possible an escapee of one of the other species is possible.
When the migrant flocks come through in both directions they tend to be pure, but do mix when feeding together. Once in flight they almost always sort themselves out completely, flying off in their own groups.
Once the main flocks passed through there are a good number of strays that show up. The Canadas tend to stay around much later, not leaving until it gets cold enough to permanently freeze the open water required for safe nighttime roosting. Local residents still seem to get by using the faster stretches of area rivers over the winter.
These stray birds seem to show up after their own species have gone on, either north or south. Annually we see mostly individual (sometimes 2 or 3) Snow Geese, and occasionally a Ross', White-fronted, and even Brant (Brent).
The heading south are almost certainly wild. There are thousands of individual snows across the country doing this. There are fewer of the other species, but I don't think people could possibly breed & lose enough of these species to account for the numbers seen (White-fronts are always a bit suspect, Barnacle Geese fully suspected as escapes) in either season.
The birds just appear to want the leadership, company & safety offered by being in a flock, and any flock seems to be all right with them.
Seems to be a minimum size for a flock though, half a dozen or less tend to align themselves with the larger group. This is probably done for the same reason individuals do it.
So, disoriented, lost and injured birds do it all the time. Escapes that migrate to a place where the opportunity presents it self to breed should become "wild", the same as released of Peregrines or Whooping Cranes.
A moulting Snow Goose in Scotland on the other hand, is just doing it for company, chances of it ever getting into a flock of its own kind seems rather remote.
