Se llama "Lázaro de Tormez" porque en el texto nos explica que su padre trabajaba en un molino harinero de agua. Entonces, un día, la madre tuvo que parir en la aceña, casi al lado del río. Por eso él se nombra "Lázaro de Tormes".
I translate the text in English for you to understand:
His name is "Lázaro de Tormez" because in the text he explains that his father worked in a water flour mill. Then, one day, her mother had to give birth in the mill, almost next to the river. That is why he is named "Lázaro de Tormes".