I think it's a combination. It's a natural consequence of both our economic and political model.
Markets that allow unregulated investment of private capital have a natural tendency to concentrate wealth through multiplier effects. Basically the more money you have, the easier it is to make money from your money, and the more money you get.
But wealth is power. As you get more money, you have more money you can use to manipulate the political system in any of a myriad of ways. Everything from donating to campaigns, to hiring lobbyists, to funding ads, to the revolving door and regulatory capture, to just straight up bribes.
So I do think there are natural concentrating tendencies that we can introduce policies, like UBI, to counter-act. But there is also pretty clear evidence of those with wealth manipulating the system to grant themselves more of both wealth and power.
Just look at the lobbying the wealthy have done over the past few decades that have taken America's formerly very progressive taxation system down to where it is today. That didn't just happen. People with wealth pushed for it. The same can be said of any number of other changes to the regulatory and economic infrastructure that primarily benefited those with wealth and harmed those with out (weakening of unions and collective bargaining, the weakening of financial regulations, weakening of environment regulations, and so forth). That stuff didn't just happen. It wasn't a natural outgrowth of the system. People made it happen.