There's also significant selection bias on the entry side. U.S. immigration policies are heavily biased towards people with unique skills, high levels of education, and a fair amount of money in their home country, and biased against people who are likely to be a drain on public assistance.
Clearly not the cause.
The USA has accepted many waves of refugees. For example the Cambodian boat refugees. They arrived with no skills, no education, no money, and didn't even speak the language. There was every reason to believe that they would be a drain on public assistance. And yet the children of these groups also show much greater upwards mobility than children of people who were already here.
Refugees are a vanishingly small portion of immigrants in the US.
There are about 47 million immigrants living in the US[1] and about 267 thousand refugees[2]. There have been an average of more than a million people gaining permanent resident status each year for the past 20 years and less than 100 thousand refugees admitted each year for the same time period[3]. Even in the single highest year ever for refugee admittance, they were only about a quarter of immigrants.
Reminds me of one theory behind the success of immigrant "model minority" groups dating back to 19th century Jewish emigrants from pogrums being their community support network and a cultural values towards education. Of course none of this is in a vacuum and depends upon the host society and reaction.
Disclaimer: Self determination and environmental determination both have their limits and maladaptive behaviors may have roots in trauma. It isn't meant to be any statement of intrinsic vice or virtue.
I could see easily how refugees forced to cooperate to survive and fleeing horrifying death would support one another when in an unfamiliar new country and knowing there is no hope in heading back.
I wonder how relevant the "hedonic treadmill" is to the phenomenon as well. Motivation and incentives having a positive impact on success is no surprise and unfortunately it isn't as easy or simple as just telling people to get motivated, making things better for them if they succeed, nor making things worse if they don't. Nor even more slippery sloped approaches like propaganda for social goods "Black Mirroring" a metric feedback loop that would be dystopian even if it worked.
But the fact that upward mobility is strongly evident among refugee groups says that a theory for upward mobility among children of immigrants should explain why it still happens despite all of the disadvantages of being refugees.
If you start at the bottom there is a lot of up to rise into and no down to further sink. Same at the opposite end: all the nepotism and Ivy Leagues in the world won't be able to keep some billionaire spawn from sinking and there are no higher ranks to rise into. Their average mobility can only be negative. I guess I'd still prefer being a failing trust fund baby over the upward mobility of rags to better rags, despite a considerable delta in sense of achievement (rags to better rags might actually be happier with themselves).
Clearly not the cause.
The USA has accepted many waves of refugees. For example the Cambodian boat refugees. They arrived with no skills, no education, no money, and didn't even speak the language. There was every reason to believe that they would be a drain on public assistance. And yet the children of these groups also show much greater upwards mobility than children of people who were already here.