lag vs drop back

lag is a noun but drop back is not a noun.

Word	Noun	Adjective	Verb	Adverb
lag	Yes	No	Yes	No
drop back	No	No	Yes	No

As verbs, drop back is a hyponym of lag; that is, drop back is a word with a more specific, narrower meaning than lag:

Other hyponyms of lag include drag, drop behind, get behind, hang back, trail.

lag (noun)	drop back (noun)
the act of slowing down or falling behind
one of several thin slats of wood forming the sides of a barrel or bucket
the time between one event, process, or period and another

lag (verb)	drop back (verb)
throw or pitch at a mark, as with coins	take position in the rear, as in a military formation or in the line of scrimmage in football
cover with lagging to prevent heat loss	to lag or linger behind
hang (back) or fall (behind) in movement, progress, development, etc.
lock up or confine, in or as in a jail