If you watch the replay, Portis ran into Matthews and knocked him down. It was a huge gaffe. However, Horford and Smart were loafing up the court because I think they were expecting Milwaukee to foul.
Tatum was wide open, but he made a major gaffe as well not coming back for the ball. If Smart tried passing to Tatum at half court then Giannis probably cuts off and intercepts or alters the pass.