If only we had the answer...
Actually we have it : suddenly, after playing great, collective basketball through 2 quarters (or 3), the Celtics decide to go one on one on every possession, run the clock down, attempt 3 pointers 1 possession out of 2, play lazy D, commit turnovers (a lot of them) and commit fouls.
Now, the real question is : why do they start doing that after halftime, and get away from what made their success in the first half?!
And to that question, I have no clue... and it looks like the C's dont have one, either.