It's a very tricky problem. An illegal immigrant to the USA has always been in the country unlawfully. However, 20 years ago they weren't pursued as vigorously as they are today. The alarm bells really started to go off around 2005 however, when a literal flood of illegal immigrants were crossing America's borders. Then the USA got more serious about the problem, and began deporting illegal immigrants in a big way under the existing laws. But in doing so it meant treating an immigrant illegally in the country for 20 or more years the same as one that just arrived yesterday. The consequence is that those long-term illegals, who in some cases have American-born children, are being deported like all the rest.

This issue was identified during the Obama administration.  He attempted to deal with it by bypassing a divided Congress and dumping the problem on individual states. His executive order to that effect was shot down in court.