The vast majority of "political" people who want open borders have a hidden agenda. Mostly it is the destruction of the status quo, in the hope that they can take over. It plays out with all of the different special interests. Many of them are also globalists, as the elimination of the United States is just a precursor to a world order of their choosing.
From the establishment side, in addition to globalism, you have the cheap labor lobby and the pyramid schemers. The pyramid schemers are sometimes obvious, as when they say that we must continually grow the population in order to grow the tax base and pay for entitlements like Social Security, and less obvious when it is the Federal Reserve who wants to suppress labor costs to offset the inflationary effects of their monetary inflation and debt.
Yes, there is often a hidden agenda at work.