It's not just the Celtics, a lot of NBA playoff teams committing silly, stupid turnovers.
It might have something to do with the playoff pressure, or the intensity of each game, causing players to just play uncharacteristically.
Warriors turned the ball over late in the 3rd quarter after tying it up at 65....gifting the Spurs an 11 point lead.
Now in the 4th quarter the Spurs are giving away the game with silly turnovers, directly jump starting the transition game for the Warriors.