Dragon fruit plants can spilt sometimes due to excess water, Physical damage, lack of support, disease or rapid new growth. 

1. Overwatering: Dragon fruit plants prefer well-draining soil and can be susceptible to root rot if overwatered. Excess moisture in the soil can cause the limb to become weak and eventually split.

2. Physical Damage: If the limb was accidentally damaged or bent forcefully, it could lead to a split. Avoid handling the plant too roughly, especially during its growing season.

3. Lack of Support: Dragon fruit limbs can become heavy as they grow and produce fruit. If the limb lacked adequate support, the weight of the fruit might have caused it to split.

4. Disease or Pests: Certain diseases or pest infestations can weaken the limb, making it more prone to splitting. Check for any signs of pests or diseases on the plant.

Rapid Growth: Dragon fruit plants can experience fast growth during the right conditions. If the growth rate was particularly rapid, the limb might not have been able to keep up with the expansion, leading to a split.

Do not worry too much these will most likely dry scar over.

If you are really worried you can apply cinnamon to the split on the dragon fruit limb as cinnamon will act as a natural antibacterial and help the scar dry. 

To prevent future splits, consider providing proper support for the limbs, ensuring well-draining soil and avoiding overwatering.

Regularly inspect your plant for any signs of disease or pests, and address any issues promptly.

If the split is severe, you might need to prune the damaged portion and take measures to promote the plant's health and recovery.