If people or things bunch up or bunch together, or if you bunch them up or bunch them together, they move close to each other so that they form a small tight group.
They were bunching up, almost treading upon each other's heels. [VERB PARTICLE]
People were bunched up at all the exits. [VERB-ed PARTICLE]
If they need to bunch aircraft more closely together, they will do so. [VERB noun PARTICLE]