Other mistake: Assunta's Ivysaur attacks Sparky too early, before the battle even started, not to mention Assunta haven't given it any commands during the battle.
Continuity mistake: When Ash and friends are after Assunta wins, a man in a blue shirt has his left hand raised, but in the next shot, it is not. (00:08:33)
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 ★