Phantom games for the GBA?

Ever so often I think back to this odd mystery glitch I ran into once or twice when I owned gba when I was a kid. It was after my console was pretty old and I never figured out what triggered it. In one instance I inserted my copy of Pokémon red version into my console, a game I regularly played and instead of pulling up my established team I ended up playing the coach bandicoot and spyro crossover, a game I didnt even own. After a while I reset the console and went back to playing Pokémon with no issues with my save file. I couldn't manage to get it to pull up the crossover again. In another instance I spent a few hours playing an odd top down rpg game I still haven't been able to identify on a different game cartridge (I dont remember which) has anyone else encountered this glitch? What caused it and how is it possible? Google ai said it was caused by bootlegs but all my games were store-bought