Why did Portugal supply weapons to the Nationalists in Spain?

Because Portugal became one of the most important allies of Spain. The prime minister of Portugal knwon as Salazar was supporting the army of Franco. Many dissidents of Portugal were protected in Spain and, even though the relations were a little bit tense, Portugal gave their full support giving aircraft as an aid to Franco in Spain. Salazar reassured Franco and his allies that no interference would hinder the supply traffic directed to the Nationals.