Well they have had failures in the WTCC, fuel pump affected Muller's car last year, and i think it's happened since on another.
Monteiro's car blew up spectacularly easlier this year, and one or two have had misfires. Technical gremlins, the diesel is still very much new to this team and they don't have the 100% backup from Spain people think they do.
One of those things sadly, just typical bad luck it happened on what may of been their best weekend of the year no doubt to score big points.