1) People who have non C.S. degrees even from good schools. Once you have finished your degree, you have three months to find a job. Some people don't find a job within those three months and are forced to leave. Honestly, I am not sure what the solution to that is? Stapling a green card on their applications post graduation seems good to help an indefinite stay but what if you can't find a job in 6 months? 9 months?
2) People who don't want to stay here. Their objective is to go back home.