Combine the onion, habanero peppers, lime juice and salt in a bowl; cover and refrigerate while preparing and cooking the pork. Tip: Use rubber gloves when preparing the habanero peppers and avoid touching your eyes, nose or skin while slicing peppers.
Heat the oil in a large frying pan over medium-high heat. Season pork with salt and pepper; cook in the hot oil until completely browned, 15 to 20 minutes. Transfer the pork to a pressure cooker.
Combine the guajillo chillies, orange juice, vinegar, garlic, and achiote paste in a blender; blend until smooth. Pour the sauce over the pork cubes in the pressure cooker.
Seal and lock pressure cooker, and cook over high heat to build pressure until the indicator sounds. Turn heat down to medium, and cook for 45 minutes. Remove from the heat and let the pressure reduce naturally. Remove the lid, transfer pork to a serving dish and shred with 2 forks. Pour the juices over the shredded pork.
To serve, top with the habanero sauce.
Slow cooker version
This recipe can also be made in the slow cooker, cooking 6 to 8 hours on High, until the pork easily falls apart. Simply add the pork to the slow cooker after browning in the frying pan, along with the guajillo chilli mixture. No additional liquid needed.