The image is fuzzy and I'm certainly no expert on this, but it certainly looks like:
1) Yes, it's probably an Argentine Ant
2) This is probably an example of a common behavior: when the nest is threatened by flooding, exposure, etc, the workers each take a larva, pupa or egg and the entire colony is moved to a new location.

Argentine ants can have dozens of colonies in an area, so the odds of this happening are fairly high. I often see colonies established on my patio under large pots and in other vulnerable locations that are easily disturbed in the normal course of gardening, which makes the above scenario quite common.