A mother's influence is detrimental to training a boy in Saotome-style martial arts, so Genma took his infant son and left Nodoka. As he left, he swore to make Ranma a manly man inside and out, or they would commit honorable seppuku (what we call hara-kiri) like men. As a martial artist's wife, she accepted Genma's decision. Truly, a remarkable woman (she'd have to be to marry a man like Genma!).
Nodoka received letters from Genma until he took Ranma to Jusenkyo, when they suddenly stopped. So she comes to the Tendo dojo, wanting nothing more than to see her son Ranma grown to be a manly man. And if Ranma isn't a manly man, she'll hold Genma to his promise. So when she hears about her son doing unmanly things like wearing women's clothes, her placid demeanor disappears and she draws her katana.