Let me assure you that running into bears at the dump is rarely a planned encounter.
Black bears are actually quite social, they just don't live in extended family groups sharing a residence. It's not uncommon for a bunch of them to live in close proximity and to get together from time to time, whether one-on-one or in small groups. Young adult bears especially form what are basically youth groups after they leave their mothers and hang around with one another so long as food permits them to.
I agree it should probably be most normal to encounter 1 bear at time, but I think between dice and reaction rolls you can pretty much rationalise a group of up to six hanging out pretty easily, even without accounting for mothers and cubs. Just put a dead moose or something on the ground and have them all snacking and hanging out, not a cellphone in sight, living their best lives.