Is behind an adverb

Yes if you look for deffinition in a dictionary, it say so.

behind /bɪˈhaɪnd/prep

1. in or to a position further back than; at the rear of; at the back of
2. in the past in relation to: I've got the exams behind me now
3. late according to; not keeping up with: running behind schedule
4. concerning the circumstances surrounding: the reasons behind his departure

adv

1. in or to a position further back; following
2. remaining after someone's departure: he left it behind
3. in debt; in arrears: to fall behind with payments

adj

1. (postpositive) in a position further back; retarded: the man behind prodded me

n

1. informal the buttocks
2. a score of one point made by kicking the ball over the behind linebetween a goalpost and one of the smaller outer posts (behind posts)