drop back vs lag

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

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

As verbs, lag is a hypernym of drop back; that is, lag is a word with a broader meaning than drop back:

Other hypernyms of drop back include dawdle, fall back, fall behind.

drop back (noun)	lag (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

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