I don't think this is about wealth, at least not likely the case with this group. Some do have hot tempers. In some parts of China, staring at a random man in the eyes for too long or try something funny is cause for a fight. I just don't think some of these men had internalised that they are in a different nation and they are in an unique situation since they are stuck in an camp isolated from the rest of society. In addition these people don't look that young, not youth by any means.
Many of the workers who are willing to take risks to better their lives and their families lives come from poor to ordinary backgrounds, most likely coming from families with multiple children. You would be surprised by the birthrate in some places, in relation to what you would expect for China. Riding on trains in China and talking to many rural folk, I have learned first hand that they tend to have multiple children. I have talked to people with 4-5 kids at home, in places like Henan. The "missing girls" theory is also mostly a myth, they are part of the hidden population, though there is a natural tendency for more males at birth.
The perception of the "poor china which was far nicer" doesn't exactly reflect reality imo. People that grew up with wealth tend to be much more educated and prefer to conduct themselves with a certain social etiquette (often nicer), though they could be cold/detached to a certain population and have some entitlement mentality. I would argue this population is much easier to reason with compared to the poor that is working on upwards mobility. The most poor are sometimes content and can be docile, while the poor that are trying to better their livelihoods and escape the poor category tend to be aggressive and motivated (relatively speaking). Many end up with that mentality since it tends to help them find more opportunities.