Cutting and changing their forward setup for inferior players is probably to blame.
Villa had a bad end to the season, but he had 20+ despite being crap since the end of Jan so he knows where the net is and surely would find his touch again.
Pedro last year was immense, as he was the year before, all this in and out the team to accommodate Sanchez who imo has been average at best has harmed the team.
Meh, a decent number of their dropped points have come when playing 3-4-3 for no real reason. Guardiola has pushed Alves forward as a midfielder and left the hapless Pique at the back a few times, Pique has probably cost them more points than any other single player.
Sanchez had picked up a bit of form lately but he also missed the best part of two months(though for me looked crap before the injury) and doesn't really suit the Barca way. Cesc has scored a buttload for him and has gotten very unlucky to not get at least a few more. Iniesta has been pretty poor, Xavi has been awesome.
Villa has broken his leg so dropping him really wasn't...... unexpected
They've had a fair few injuries, and Puyol hasn't been close to his best which really leaves Pique looking like a bit of a joke.
Realistically they are a team that really hasn't faced "real", or Real pressure for a few years. They were champions and solid as a rock when he took over and though Real came closer, they were basically ahead the whole season with the better head to head record and really never in danger.
Guardiola tried a few things, lost a few points, Real are stronger than last year, so going a few points down its really the first league pressure they've faced for 3 years and they are now playing like a team not as relaxed or freeflowing as they were.