I would say that many atheist are secularist also. Unfortunately these terms are seen as one but definitely represent fundamentally different viewpoints.
Atheist: Simply means; a person who does not believe in a god/gods. The reasons are varied and cannot be nailed down to one point. (By the way, it is not a religion either. * insert blank stare here*
Secularist: A person who support the principals in secularism, the separation of government institutions and persons mandated to represent the state from religious institutions and religious dignitaries. (Yes, I copied from Wikipedia)
Using these definitions, it seems obvious that they need not be applied together to an individual or group. The propensity for people to assume others beliefs based on one aspect of their personality seems to be a common trait in the human condition. (I know that I a guilty of this but I will admit this could be confirmation bias.) It suggests an oversimplification is made when assesing others when part of a group and leading to generalizations that fit the narritive already established by the one piece of information. This can be a useful shortcut in reasoning when the assumptions need to be grouped together to be true. Yet, without skeptical assesment, this shortcut becomes fallacious thinking.