Cracking your back is relatively safe, but can come with potential risks when you do it yourself:

• Misalignments: Improper self-adjustments can worsen existing spinal misalignments or create new issues. 
• Overstretching Ligaments: Repetitively cracking your back could overextend the supporting ligaments leading to instability. 
• Nerve or Muscle Damage: While rare, excessive force could irritate the nerves or strain muscles.