It's a personal thing & it's very uncomfortable if you happen to be afflicted with the sensitivity. Think about how you'd explain what causes motion sickness to someone who never gets sick and doesn't know anyone who does. From what I understand, the noise canceling effect conflicts with what the user is actually hearing from multiple sources (i.e. the iPhone earpiece, background noise in the call, ambient noise in the room the user's in.) It messes with their brain. Apologies for the simplistic explanation; there are fancier medical ones.