The reason why both Kevin Owens and Sami Zayn were sent back home from the WWE European tour is because they did not go with the script on Smackdown and took matters in their own hands.
Ryan Satin of ProWrestlingSheet.com reports that following the match of Sami vs Kofi Kingston, the two were supposed to stay in the ring and get beat up by The New Day. After the match ended with Kingston pinning Zayn, Kevin Owens hit the ring and attacked Kofi. Rather than staying in the ring as planned, both Superstars then bolted and walked to the back.
Veering off the script did not go down well with those running the show backstage and the decision was taken to send them back home as a disciplinary measure.
Neither Owens nor Zayn has publicly spoken since the incident.