Population pressure...but the other side of the coin is the extinction of megafauna on which many human societies had depended.

As in California, the end of megafauna hunting meant an intensification of dependence on other resources, including small game and plant foods. With the mammoth and horse herds gone, people ate rabbits, acorns, etc.

As for the discovery that seeds planted would sprout, the best theory I've heard is that it was in the garbage heaps which accumulated near any settlement with any permanency. These middens provided a fertile soil (compost, basically) in which for the seeds thrown out with the "garbage" to sprout, and were conveniently close to dwellings so that all could see what was transpiring.

The date for corn in Mesoamerica is far later than the first domestication of food crops. Domesticated tomato remains are reported for Tamaulipas (northeast Mexico) from 9000 yrs. bp. Most of the other major Mesoamerican domesticates predated corn as well. There was a long "Archaic" period of town settlements and agriculture which took off as what we label "civilization" only when corn was domesticated, at a date which, as Michael Coe points out, coincides with the appearance of the "long count" calendar among the early Olmecs and/or Maya.

The knowledge of how plants "work" was no doubt available long before domestication occurred. It has been observed that in fact California Natives were tending to wild plant stands in order to improve the quality of their produce as well as enhance their growth. An example is the coppicing of sagebrush to produce longer wands for burning. Another is the intentional enlarging of piñón (pinyon, if the accents won't show up) groves by planting the seeds, a practice observed among the "non-agricultural" Paiute in the hills bordering the Mojave Desert.

I do ramble a bit, but the main point here is that much of the knowledge of plants and their growth already existed, early forms of agriculture which involved the tending of wild stands were already practiced, and the changing climate which produced a more favorable environment for "farming" entailed also a drastic diminution in the availability of large game species. Given all these factors, it does not seem at all surprising that agriculture arose nearly simulataneously in so many places in the early Holocene.