I would think that it would have been more gradual than that. Some people in the community probably started growing the plants that they usually went out and gathered, since it was easier. Maybe during a time when they weren't moving as much. Or maybe they migrated between different areas depending on the season, so they planted, say, berries at all the different sites. I think it would have started more like that. Then because there was more food available from this, more people started doing it, and eventually it won out over migrating all the time or even staying in one place and hunting and gathering.

The wild rice gathering practice of current-day Native Americans in Minnesota is a great example of incremental drift into agriculture. They go back to the same patches in the water brush rice into their canoes with paddles, inadvertently spreading the seeds of new rice plants in the process. It's easy to imagine how this could've turned into rice agriculture over time.

Could be because life was better in the (relatively) short term, but ended up worse overall. Maybe it also seemed better than it actually was. Evolution had prepared us to see and fear the hazards of hunter-gatherer lifestyle (such as hunger), but not of agricultural lifestyle (such as eventual disease).