As gofast182 indicates, I say it's strategy. Simone Resta wasn't all that essential, and this was done with eyes open. I was concerned about James Allison initially, but that was long enough ago that he had no bearing on this year's car in the first place, which has been doing well as it is. James Allison also wasn't sacked. His wife passed away and he needed to be closer to his family, rather than in Italy.
I'm not sure Vettel would even be leading the championship right now had he not made any of his mistakes. The strategy blunders, forces majeure, and the lack of pace on certain tracks alone have been enough to get Hamilton ahead. Of course, it wouldn't be handing it to him on a platter like it is now.