As you hopefully know, Pokemon can come out on their own, as seen when Ash's Oshawott or Brock's Croagunk comes out without their Trainers doing anything. Also, as stated by Bulbapedia, Section "In the Anime",(https://m.bulbapedia.bulbagarden.net/wiki/Released_Pok%C3%A9mon): As shown in 'When a House is Not a Home!' (JN59), Pokemon can even destroy their own Poke Balls.