The Williams race recap (a few posts ago) pretty much says they got screwed by the yellow/red flag for Kevin Magnussen on lap 6.
Not sure why they pitted Massa so soon, but all other cars essentially got to "cover" the Massa strategy by stopping under the unexpected yellow on lap 6. Massa was screwed to the back of the line.
Then Williams correctly predicted a red flag for the Magnussen crash, but they incorrectly predicted it would occur immediately. Thus they left Bottas out waiting for the red flag while the FIA dithered under yellow. All other cars nailed a nearly-free yellow stop before Bottas finally came in, and was screwed to the back of the queue.
I'm glad Magnussen was OK, but as a Williams fan that Magnussen crash was tough for me, and to make matters worse I had picked Magnussen in my office pool (have to pick every driver once)