Why does the Torah write "B'nei Naftali", and not 'li'Venei Naftali', like it does by all the other tribes?
Ba'al ha'Turim: Because the tribe of Naftali had more women
It is not clear as to why "B'nei" has connotations of women more than "li'Venei". See Ba'al ha'Turim.
And that explains why in Parshas Pinchas, the Torah writes "B'nei" by all the tribes, since at that stage, the men had died, and every tribe comprised more women than men.