Baneboo's Lost Object!? - S7-E39
Continuity mistake: When Ash tells Corphish to use Bubble Beam after Team Rocket's Pokémon during the first battle, his bag disappears. This also happens when Ash tells Torkoal to dodge Cacnea's Needle Arm. (00:07:52 - 00:19:02)
Answer: There are a lot of fan theories around this, but the most common theme is that Pikachu had another trainer before Prof. Oak, who gave him to Ash. Pikachu didn't appreciate being handed over.
Ssiscool ★