Why are antipsychaotics like compazine (prochlorperazine) often termed "major tranquilizers" when they often cause akithesia and anxiety and do the opposite of tranquil?

Good point: In psychiatry, we call those medications antipsychotics, because they are usually used to treat psychotic symptoms. We don't use the term tranquilizer, because it is misleading. As far as your question is concerned, the side effects you mention are unintended, so they are not part of the intended use. As for anxiety, i'm not familiar with the antipsychotics causing anxiety.