Your cartridge is possessed! CreepyPasta!

But no, seriously, it sounds like an issue with the hex values on the cartridge. There is information being mistranslated between the initiation of internal commands somewhere, probably due to an action that you have taken in the game, which technically wasn't supposed to happen, therefore causing data with similar values to also become garbled. I highly doubt it would be your console, seeing as the problem is specific to that one game, and that it only happens at a specified point.

To put it simply, your game is glitched. If you keep playing and save in a different place, it may eventually work itself out, or it may be associated with that save file specifically. You could try starting all over to see if it is just that particular save file, which is most likely what you will have to do. Worst case scenario, if it isn't just the save file, the cartridge itself is corrupted. You would have to get a new one, in that instance.