On his personal site he stated that car was unstable in turns (sliding). Since that happened to me too once (not in F1), I know precisely how that (s***e) feels.
from Mark Hughes article:
"What happened in the top-10 shoot-out was that Schumacher messed up his one lap and Rosberg did not.
Schumacher took too much speed into Turn One - just as he did in 2006. That pulled him out wide on the exit, he got dust on his tyres and that impacted upon the rest of his lap.
"Looking at the data, we can see Michael had considerably less tyre grip through most of that lap than he'd enjoyed previously," said team principal Ross Brawn.
"It takes a long time for the tyres to recover when you get off line."
That was what was responsible for probably around 80% of the one second chasm between the two Mercedes drivers in their final qualifying times.
Because the team had committed to single-lap runs, that deficit was frozen and goes down in the records.
Yes, it was Schumacher who ran wide and Rosberg who did not, suggesting perhaps that Schumacher's pace was coming with less ease than Rosberg's, but the genuine performance gap between them had they each performed to their potential was in the order of 0.2secs - around half the gap between the Red Bull drivers on the front row, for example."
Maybe going off he got dust and marbles on his tyres.
I personally after the race in Turkey picked some marbles for myself from turn 1. I think they become fans attraction.