No, there's no such rule, but I wouldn't have given it. Was Fàbregas tripped, or did he trip over Targett's leg. I've watched it again at home a few times and it certainly looks as though Fàbregas is already going down and tries to make the most of it. His reaction suggests to me that the ref was right. On this occasion at least. The rest of the game he was in Chelsea's pocket.