Serebii Doesn’t explain why moves become Plus Moves but it only happens automatically to moves that are in a Pokémon’s Natural Learnset, usually in an interval of 5-10 levels after they learn that move. Whether that’s a Level Up timer attached to the move itself (that doesn’t work for moves learned Via TM), or Pokemon have a seperate learnset for plus move availability is unknown.
Of course, you can use Seeds of Mastery with an NPC at the Justice Dojo to turn any move, Natural or TM, in a Pokémon’s moveset into a Plus Move.
And Mega Evolved Pokémon treat all of their moves as if they were Plus Moves. :P
Source; Exp
Hope I Helped!
