If you're a straggler, you tend to dawdle at the very back of a group, often becoming separated from them because you're moving so slowly. Stragglers are constantly in danger of missing the bus.
A straggler is the last person to leave a party, lingering to finish their cake and wander around looking for their coat. On a field trip, chaperones worry about losing stragglers who keep wandering away from the group. Straggler and straggle probably share a root with the Norwegian stragla, "walk laboriously." In astronomy, a blue straggler is an unusually luminous star originally believed to have formed later than the other stars in its cluster.