There is a difference of opinion regarding the translation of this verse, and therefore also in regard to the meaning of the words. But staying close to the original text gives a logical explanation. David asks if the evil that his enemies wish upon him may come upon themselves. In Psalm 140:7, David spoke about his head being protected—now he requests that the head(s) of his enemies may be hit hard by what they did to him with their words (for the poisonous influence of words, see Psalm 140:3).
9 As for the head of those who surround me, let the mischief of their lips overwhelm them!