Usually it's based on your point of view to begin with. Everyone believes their beliefs are reasonable, logical, and therefore centrist, since obviously their ideas are the most balanced and reasonable. So from that point of view they see anything that doesn't fit their own view as either left or right from there. Hence the view of many on the extreme left that politics has "shifted" so that centrists are the "new right", rather than being intellectually honest and admitting that their own view have shifted extreme left and become even more immutable. We are in an age where confirmation bias is so strongly reinforced through immediate access to similar viewpoints and ability to deride dissenting opinions through the immediate feedback loop of social media, that even those who may previously have accepted other opinions as valid are becoming engulfed in the political quagmires at each end of the spectrum.