lag vs drop behind

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

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

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

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

lag (noun)	drop behind (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 behind (verb)
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