Because they had found no answer, and yet had condemned Job - They held Job to be guilty, and yet they were unable to adduce the proof of it, and to reply to what he had said. They still maintained their opinion, though silenced in the argument. They were in that state of mind, not uncommon, in which they obstinately held on to an opinion which they could not vindicate, and believed another to be guilty, though they could not prove it.
Related Commentaries of Job 32:3
Job 32:3
3 Also against his three friends was his wrath kindled, because they had found no answer, and yet had condemned Job.