This was completely on the players.
In the last 15 seconds:
Kemba seeks the ball in the backcourt along the sideline, which is a huge no-no.
Hayward is open in front of him in the middle for an easy entry, but Smart locks in on Kemba anyway.
Tatum completely botches the last play. It was not a complicated situation. He has a 6'0 player on him, with Kemba's man hedging. No one else is in a position to help.
He doesn't force the help defender to decide and doesn't get into the lane, and he takes a difficult fall away as if he's being covered by Giannis.
Really poor decision making all around.