It can be see that the elastic constants you offer contain a pretty high C33, which is 3468 GPa. It means that the Young’s modulus at the direction Z of this material is more than 1800 GPa according to the equation
Yes, agreed this doesn’t sound very sensible. Note we do have warnings associated with this elastic tensor on our website also:
As for the reason, I’m not sure. We’re currently looking for a team member to help out on some elastic tensor maintenance, but at present we cannot investigate further for this specific material.