Yes. Sorry that this is not explained in-game at the moment.
Forests get turned into cities when humans manage to place 2 cities adjacent to them. So placing forest tiles with flanks exposed can quickly lead to a chain reaction in which whole groups of forests get flipped to cities in a single move. Forests are like a phalanx. They hold the line, but if they get flanked, it can quickly lead to a complete rout.
Mines and Farms each provide one extra move to the humans on their next turn, and can be placed within 2 tiles of another human tile. So a chain of mountains can rapidly be covered by mines, and a river with farms.
Perhaps I should nerf the humans' ability to place mines and farms so that mines and farms don't count as cities for the purpose of placing additional mines or farms.