Ace was always far stronger than his littermates, but it wasn’t a power that he was good at controlling. Despite this, Pika and his siblings believed that he would be able to control it some day and Skye even named him “Ace” to show him he’ll become an ace Pokémon.
When he evolved into a Pikachu, the problem persisted. Skye’s parents were worried, as he was more stronger than before and being unable to control that power was. . .bad.
Pika and Skye tried their best to help him through it, but that typically meant keeping him within reach, which made him restless.
One day, when he was having a bad day with his electricity so he needed to get his extra energy out. He tried playing with other Pokémon, but the built-up electricity made him irritable, so after he gets thrown around by bigger and older Pokémon--he becomes angry.
He released all of his pent-up electricity in a single bolt, destroying a nearby tree.
Skye’s parents decided that they couldn’t keep Ash at the ranch anymore, as they believed that he was too much of a danger to keep him around so many other young Pokémon. So, they decide to give him to Professor Oak until he could control himself.
When the decision was made, Ace thought that they just wanted to get rid of him so when Pika and Skye tried to explain what was happening, he refused to listen and ran off.
And after his parents told him to catch him, Skye followed and captured him in a Starter Pokéball (which is inescapable for Pokémon under level 5).
Inside the Pokéball, Ace panicked and continued to do so until he was released at Professor Oak’s lab. His memories of Pika, Skye, and the ranch were repressed (and simply forgotten due to leaving the ranch as a young kit), and replaced by his fear of abandonment and being inside of a Pokéball.
Before giving him over to Professor Oak, Skye stuck a small lightning bolt sticker out of fear that the professor will misplace him or his Pokéball somehow.
Of course, Professor Oak doesn’t misplace him. Because he realized that Ace was the Pikachu he met in the future, so he gave him over to Ash as his starter Pokémon.