Atlanta has unique style of defensive pressure, they over play and challenge the ball handlers. This calls for a response that either the celtics couldn't execute, or the coach didn't call for.
The Celtics, to a player didn't come to the ball to catch a pass, to create space when being over played. They instead waited for the ball to reach them, many times too late.
Atlanta has a nice team, they play hard and smart, that defensive style is causing havoc through out the league, opposing NBA players hate to be pressured, plus the Hawks have some knockdown shooters. There not real big, but quick and fast, they really know what they do best.