The city boundry sneaks up on the 20 acer farmer and he is then included in a "sphere of influence" or within the city limits themselves. This imposes additional restrictions and costs on his operation.
A developer then offers the struggling, small farmer a substantial, up front, cash offer, for an option to purchase his land at an inflated price at a future date. Typically about $2k per acre cash for an option to purchase in two to five years at triple the going rate for ag land.
When the option deadline nears the farmer realizes that he can get ten times the ag value but too late. He has to settle for the "reduced" rate. Continued farming is not an option when surrounded by homes, shopping centers, the neighborhood kids and a metered water supply.