Hi!

Most of the spanish population has traditionally lived near the coast, so crossing mountains was never a factor for most potential migrants. Instead, the main driving factor for Galician migration to the americas was poverty, which was intrinsically linked to the galician economic system at the time.

You see, while in southern Spain lots if land is divided in large latifundiae (large swathes of land owned by a single family, usually noble) where people work the land in exchange of a salary (this continued to this day) and in Catalonia there was a system where many families owned small to medium estates called "Masia/Masies" where land was exclusively passed down to the eldest son (l'hereu) and most other sons learnt a craft or became priests, driving urban populations up and diversifying the economy, Galicia had a different system.

Land in Galicia was divided equally between all male children, regardless of the size of the plot of land had. Over time, this resulted in plots of land so small (minifundiae) that couldn't even support the family that owned the plot. As Galicia was a very rural area until recently, this meant that there were a lot of galicians who couldn't support their families working the land and also had no industrialising urban settlement so large that could accomodate them and offer them work.

Lots of people were stuck and famine ensued due to the situation and what industry tgere was being not competitive enough. This unique combination resulted in many men taking the trip to the Americas and later paying for their families to join them there. There being so much of them, them being a separate group from castilians, and them speaking their own language, made them very noticable.

Sources:

Malheiro Gutiérrez, X. M. (2013). El fenómeno migratorio en Galicia: del obrero emigrante al ciudadano acogedor… ¿lecciones aprendidas?. Migraciones. Publicación Del Instituto Universitario De Estudios Sobre Migraciones, (31), 155-178. Recuperado a partir de https://revistas.comillas.edu/index.php/revistamigraciones/article/view/925

Núñez, Xosé-Manoel. “History and Collective Memories of Migration in a Land of Migrants: <em>The Case of Iberian Galicia</Em>.” History and Memory 14, no. 1–2 (2002): 229–58. https://doi.org/10.2979/his.2002.14.1-2.229.

Renée DePalma (coord.), Antía Pérez-Caramés (coord.) Galician Migrations. A Case Study of Emerging Super-diversity. Springer USA. 2018. Migration, Minorities and Modernity, 3. ISBN: 978-3-319-66304-3