Here is the series of tweets. They comment on why the Flagrant 2 and ejection but not why the technical.

Explanation (1/3): 34.3 seconds left/Q2: Horford threw an unnecessary and excessive forearm/elbow to Dellavedova, making contact.....

— NBA Official (@NBAOfficial) May 25, 2015

https://twitter.com/NBAOfficial/status/602652623846645760

Explanation (2/3): 34.3 seconds left/Q2: ... Above the shoulders, therefore a Flagrant 2 foul was called on Horford...

— NBA Official (@NBAOfficial) May 25, 2015

https://twitter.com/NBAOfficial/status/602652806844182530

Explanation (3/3): 34.3 seconds left/Q2: ... Who was ejected from the game, and Dellavedova received a technical.

— NBA Official (@NBAOfficial) May 25, 2015

https://twitter.com/NBAOfficial/status/602652859931439105