Win too much for too long, and folks will try to knock you down. Maybe it's envy, maybe it's a hunger for something new or maybe it's the arrogance that comes with success.
Since the summer of 2008, when Pep Guardiola took over at Barcelona, the club has won 13 of 16 possible trophies. It's also still in the running for another three titles this season: Barcelona has reached the final of the Copa del Rey, the semifinals of the Champions League and is currently in second place in Spain's Liga, though catching Real Madrid would be very difficult.
So perhaps it's not surprising that the ground is fertile for some kind of backlash against Barcelona. Apart from one notable exception, though, there hasn't been much at all.