What is normal? Normal is conforming to a standard; usual, typical, expected. And what could be more normal than the humble normal distribution?
The normal distribution, also called Gaussian distribution after German mathematician Carl Friedrich Gauss (the Duke of Mathematicians) is everywhere, from the social sciences to the natural sciences to machine learning.
So it was only natural for Charles S. Peirce, writing in the late 19th century, to call it normal, referring to its widespread use and usefulness.