Firstly, there is no reason why an adjective should not be placed after a verb. In e.g.
He was dead.
you'll note that the adjective 'dead' follows the verb 'was'!
Your question presumably refers to the fact of an adjective following a non-copular verb. This can occur in cases where the speaker wishes to describe, not the manner in which the action of the verb is performed (in which case of course an adverb would be required), but the state/condition of the subject upon completion of the action.
He escaped unharmed.
means 'When he escaped, he was unharmed'.
A similar form of complementation can occur with nouns, e.g.
They parted friends.
(=By the time they parted, they were friends).