Usually, a respectful son will walk humbly behind his father. The son demonstrates his respect for his father by letting him go first.
However, if the path is muddy or blocked with branches, the son will insist on going first. This way he can clear the way for his father to follow.
Protecting Their Father's Honor
When Shechem and Chamor came to speak to Jacob about Dinah, something unusual took place. Jacob was silent, and his sons spoke in his place (Gen. 34:13). Why did they not show more respect for their father?
Ordinarily, Jacob's sons would never have spoken instead of their father. With the incident of Shechem and Dinah, however, they wanted to save their father from having to be involved in the case; so the sons spoke.
Therefore the Torah emphasizes that the brothers spoke in mirmah. That was the reason they felt it necessary to speak in their father's place. They were like the respectful son leading his father through a muddy path, saving their father from having to deal directly with this sordid business.
Adapted from Mishlei Ya'akov, p. 81.