cluster vs bunch up

cluster is a noun but bunch up is not a noun.

Word	Noun	Adjective	Verb	Adverb
cluster	Yes	No	Yes	No
bunch up	No	No	Yes	No

As verbs, bunch up is a hyponym of cluster; that is, bunch up is a word with a more specific, narrower meaning than cluster:

Other hyponyms of cluster include huddle, huddle together, bunch, bunch together.

cluster (noun)	bunch up (noun)
a grouping of a number of similar things

cluster (verb)	bunch up (verb)
gather or cause to gather into a cluster	form into a bunch
come together as in a cluster or flock