Sidewalk Spread
The maneuver the slow-moving group ahead of you does to keep its advantage when the sidewalk widens out. Instead of holding together and letting you pass, they widen their group stance to continue taking up all available space from left to right.
You're running to the bus but a family with strollers, a shopping cart and an elderly person's walker is blocking your way. You think, "Great, I'll pass them on the right as soon as we get past that fire hydrant and the space on the right opens up." But when the family gets past the tree, sidewalk spread kicks in: the shopping-cart pusher veers right, and now the whole family is still forming a wall of people, completely blocking the sidewalk from left to right.