It has to be a paper feed issue (Printer cannot print crooked, the paper has to feed crooked).
But I would have no way of saying what roller or gear is at fault.
I have a 6600N A4 Colour laser and have just noticed it doesn't print square on the paper. I haven't used it much and going back through my older prints, see that it has always been like this.
Any ideas on what is causing it? I've now tried several different papers, including Xerox and it is the same on all of them.