I don't know much about legality, and I admit that freely. I also grant that there are multiple definitions of invasion. I am using this one: "an unwelcome intrusion into another's domain."
The immigrants are unwelcome: They are not coming through the already defined legal immigration process.
The immigrants are intruding: Since they are unwelcome, and they are entering our lands, it's safe to say they are intruding.
America, from their perspective, is another's domain. Thus, by that definition, they are invading America.
I call them an invasion force to emphasize the size of the intrusion. I would label every instance of illegal immigration technically an invasion force, but this is a much bigger group of immigrants.