I doubt it would be personal, I think it would just be that perhaps Raf or his casting directors don't see her as a great fit for Dior right now, like they would with many other models. Personally I think she would've looked better out there (at Dior) then most of those other girls but that's just me..
She definitely isn't flavour of the month anymore, not that she ever really was, but that doesn't mean that she can't still book great shows.
I do find it odd and a little sad that she wasn't there, Raf seemed to really embrace Nicole but it seems lately not so much.
I think if anything it just showcases the fickle nature of the industry. It's rare to see loyalty, designers are all about a model one minute then the next it seems they couldn't care less.
Raf has his favourites (Daiane, Daria and of course Kinga) who he shows loyalty toward which is nice and I did think that Nicole was another girl who he championed as well but maybe not anymore.
It could also just as well be her height too, I think that's a realistic factor considering just how tall she is...