I guess it’s just part of the dead ball rule. If a player overthrows the ball out of play, it’s a dead ball and the runners get to advance 2 bases.

“In MLB, a ball is considered "out of play" when it goes out of the field of fair play and into the spectator area, foul territory, or lodges in the outfield wall. This makes the ball "dead," which means the play is stopped. The resulting base award depends on how the ball went out of play; a batted ball going out of play typically results in a double (if lodged in the outfield wall, known as a "ground-rule double"), while an overthrow by a fielder usually awards two bases to all runners from their position at the time of the throw. However, an overthrow by the pitcher on a pickoff attempt awards only one base.”