The card wording and oracle wording are subtly different. The card says
Pay BBB during your upkeep or the Hordes becomes tapped and you lose a land of opponent's choice.
The oracle wording says
At the beginning of your upkeep, unless you pay BBB, tap Demonic Hordes and sacrifice a land of an opponent's choice.
So according to the Oracle wording, you sacrifice a land of an opponent's choice. Now, I don't know if the MtG rules explicitly say "When an effect that you control refers to an object or players when more than one would be valid, you choose which one it refers to", I think it falls under common sense. If a card says "Sacrifice a permanent you control", it means "Sacrifice a permanent you control of your choice". While my cursory search of the rules didn't find that particular principle articulated, it did find this:
602.3. Some abilities specify that one of their controller’s opponents does something the controller
would normally do while it’s being activated, such as choose a mode or choose targets. In these
cases, the opponent does so when the ability’s controller normally would do so.
602.3a If there is more than one opponent who could make such a choice, the ability’s controller
decides which of those opponents will make the choice.
Normally, when you are instructed to sacrifice a land, you can choose which land to sacrifice. Here you are being told to allow an opponent to choose, so 602.3a tells you that you can choose which opponent will make the choice.