well, you have to ask why some of them took it up. but once that initial tribe did, they could've spread their way of life by conquest, since agriculture generally supports bigger populations than hunting. that newfound power might answer the why-keep-it-going question. as to why they'd even start (assuming they didn't decide to switch based on theories about population growth), i recall hearing a plausible-sounding hypothesis that hunter-gatherers experimented with agriculture to keep rare drug-producing plants in supply.