There are quite a few assumptions being made here. Was the flight in question a connecting flight? Was their previous flight delayed? Was their previous flight delayed enough that they missed their original connecting flight and needed to be switched onto a different connecting flight at the last minute? When my wife and I travel together, we always check in early and always book seats together. We do all the things you're supposed to do to make that happen like asking the gate agent to try to seat us together, asking the gate agent if purchasing an upgrade will allow that happen etc and yet we still sometimes end up on flights where we're not seated together. I guess we're just cheap and lazy and its always our own fault if we're not seated together though.