The main reason for the resistant palmer amaranth is herbicide overuse, especially the widely used glyphosate (RoundUp, commercially). When we depend too much on any tactic for killing weeds—or mosquitos or bed bugs or any other pest, for that matter—they develop resistance. For palmer amaranth, this has been exacerbated, in part, by herbicide-tolerant GMOs, which I've written about here. Although the resistant palmer amaranth is sometimes called a superweed, it's important to note that the genetically-engineered genes have not jumped from GMO crops to the weeds; instead, this is just regular old evolution responding to the pressure of chemical herbicides. It's a problem that American farmers have had even before GMOs were introduced, as they fought a variety of herbicide-resistant weeds in the 1980s and 1990s, too.