Sure, career progression and the new guys playing well is a factor, but this is essentially the exact same team from last season and the playoffs. That's not to say they weren't good. They went 53-31(my numbers may be off) after the All-Star break including the playoffs last season. It's like once they realized how good they could be the light went on, but they haven't gotten complacent.
Now they just come out with big swinging dicks every day and impose their will on opponents.
It's really a treat to watch. Of course they'll hit a rough patch with an injury, and maybe the offense will cool suffer some extreme cooling eventually, but there isn't any reason to believe that they can't keep winning at a very high level the rest of the way.