It's as simple as Afonso Henriques losing as couple of battles in the first decade of the 12th century.
[Actually, not, another of my old projects was a TL where Galicia became the other independent nation in Iberia instead of Portugal, but I abandoned it because I never got a plausible enough PoD]
Or even the OTL Kingdom of Galicia living on and expanding south like its neighbours (it would be an interesting TL).Well, you can always try some weird stuff, like the Normans invading Galicia instead of England and stablishing a kingdom for them.![]()