If a group is straggling, it's spread out and moving slowly. A straggling group of students going inside after recess might need to be herded and encouraged to move toward the door.
You might watch geese flying south for the winter, moving in perfect formation, and then far behind spy a straggling group of less organized birds. When something's straggling, it is lagging behind and sprawling out in various directions. It comes from the verb straggle, or "wander from the proper path" as of 1400. It's believed to come from a Scandinavian root, related to the Norwegian word stragla, "to walk laboriously."