The girls resisted detainment, then physically assaulted the officer. Then, resisted arrest.
The cop probably made it clear the girls were being issued a citation for jay-walking, which, they evidently tried to escape. Hence, his attempt to physically restrain them.
That's a tough situation. The Cop had no backup. Both hands were tied up, holding one girl and fighting off the other. Surrounded by an unsympathetic crowd. The cop seemed unsure of himself. He had every right to use more severe force since the girl assaulted him.
Very stupid on the part of the two black girls.