A plural verb may be used when stress is laid on the fact that the neuter plural subject is composed of persons or of several parts:
τὰ τέλη τῶν Λακεδαιμονίων αὐτὸν ἐξέπεμψαν
φανερὰ ἦσαν καὶ ἵππων καὶ ἀνθρώπων ἴχνη πολλά
a. With the above exception Attic regularly uses the singular verb. Homer uses the singular three times as often as the plural, and the plural less frequently with neuter adjectives and pronouns than with substantives. In some cases (B cross135) the metre decides the choice.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].