It's only ever happened to me once, and as fields suggests it was because the wrong rider (who had the same name!) was picked up. In my case the pickup was near an event and I wasn't able to get through when I tried to call. I only got about 1 minute up the road before the correct pax cancelled. Lesson learned. Nowadays if I can't get through on the phone near an event or large crowd I'll ask them to show me their Uber app to confirm I'm their driver. If they can't do that then I won't take them.