Most enter legally, and then something happens which changes that.
For some it's government maladministration. Outside their control. This happens a lot. I think that's inhumane because it's outside their control and they don't have the money or legal backing to fight it, and because the system does not apply ordinary principles of justice to resolve these issues.
For some, it's abuse by government. That is, something just plain illegal by the government, not a mistake but an intentional administrative action which is against the law. Again, I think that's inhumane because most people don't have the money or legal backing to fight.
For some it's that their partner's wage drops below a threshold. Outside their control. I think that's inhumane because once you are in and settled and started a family etc. you should not be so precarious that a slight change in your partner's income results in you having to leave.
Think of the number of people during the pandemic whose income has reduced. Now imagine the added stress of facing deportation on a plane because your partner's income dropped.
For some it's losing their job. Outside their control. I think that's inhumane because it's a breeding ground for abuse by their employer: "if you don't do whatever we tell you and suck it up, we will fire you and you will have to leave your family and the country".
For some it's their partner dumping them. Outside their control. I think that's inhumane because it's a breeding ground for abuse by their partner: "if you don't do whatever nasty things I demand you do for me I will dump you and you will have to leave your job and the country".
For some they never entered - they were born in the country. They are legal until age 18, at which point they lose rights.
I think it's inhumane to remove someone who is born and bred in a country, and who doesn't even know something will happen until at the age of 18 or so, they apply to university, and find out not only can they not go, they get a letter telling them to leave the only place they have ever known. Go "home"... where? There isn't another "home".