I'm not totally sure, but I did a thought experiment and realized that if I lived in another country I might have a pro-incumbent voting bias too. A combination of a feeling of "maybe I don't have all the information by not having the experience of living here" and "I'm not one of them, this is their choice, not mine." Or something. Maybe I'm just crazy. But it's what came to mind.
This explanation is probably correct when it comes to immigrant communities. I know for a fact that if I were to immigrate to America, I would pretty much never take part in any protest or really, anything political because I'd feel like I was interfering in other people's problems. And even after gaining citizenship, I would probably have a strong incumbent bias too.
But the explanation doesn't work for the black belt, native reserves or historical Hispanic populations ( like those in the rgv) as they've been part of the country for generations (or even since its inception).