If you don't set your combine adjustment properly (and sometimes even if you do) you throw good corn seed out the back. This is the primary source of "volunteer" corn. These seeds will sit on the soil surface, or get mixed into the soil with tillage, and grow the next year. Because corn seed has no (or little) dormancy, the problem goes away after that. Just image what cool new herbicide resistant weeds we are creating with biotechnology: IMI-R, SU-R volunteer corn, not to mention Round-Up Ready volunteer corn.
Here is what the source of the problem is, volunteer corn seed (below).
Volunteer corn is well-adapted to many habitats, including soybean fields (below).
In barley (below, left) and rapeseed (canola) (below, right).
In waste areas with other weeds, especially wild carrot (below). These pictures were taken in a field in Michigan that someone didn't harvest. Below, center, you can see two generation of corn: the old gray plants with a corn cob and seeds visible. In Iowa not harvesting a corn field seems crazy, but if you got a job in an auto factory in Michigan, and like to hunt pheasant after work in the autumn, it makes a bit more sense.
Below is a picture I took along the railroad tracks in Moscow in 1992. Amongst the trash and anguish of that city I found some volunteer corn plants doing quite fine, thank you. Weeds will out.