I think anything can cause it to happen. From the wealthiest who nose-dive into it because of horrid decisions, to those born into it. I think most people are able to find a way back out if they didn't start there. I think people born into it will find it much more difficult.
The best, most general answer I can give for why some people never seem to make it out of poverty, is that some people are unable to assimilate into and succeed in the society they belong to. In the US, some real knowledge of self-worth, a decent work ethic, a dash of self-confidence, and some knack for managing money are needed to really climb the ladder of socio-economic status.
I do fear that, these days, many of the factors for success in our society are becoming hereditary. In the IT world, for instance, where nerd-marriages are common, the rate of Autism and Asperger's Syndrome in the children created by these marriages is much higher than normal. It could also be that, as the older poor teach the younger poor, the skills and knowledge that prove most valuable in this society are fading.
We also tend to look down on those who have little, thinking what they have is a result of their actions, when often, the result is from external events. But the bias is against them. We coddle the wealthy, and expect the poor to work harder for less. Because they apparently are expected to prove themselves to those higher up the chain. We live in a society where wearing nice clothes, being sociable, and knowing the right people can get you as much, and often more, than an unknown in jeans, who isn't a socialite.
There are many more reasons I'm sure, but these are just some I thought up right now.
The root cause of poverty is not having enough money.
The other would be bad circumstances beyond their direct control. No one is like "I want to be poor." If they lack a work ethic or have some other character flaw, that's still not their direct control.
Well, let's see. The poorest state in America is Mississippi. By a LOT. Along with Arkansas and Alabama.
The three richest are Maryland, Connecticut and New Jersey. At least up to 2009. How are these states different from eachother? Guess which ones have the highest taxes? Guess which ones have the best education? Guess which ones have the best healthcare? The highest paying jobs? So why is Mississippi so impoverished? Yeah I know you want to try and get people to talk about work ethics and irresponsibility to try and take it down some conservatard argument. But let's look at the facts instead. Why is there so much poverty in most conservative parts of the country? Why is the deep south the least popular place in the country for people to live and the least likely place for people to move to? Think about it.