Form elements validation order
hook_module_implements_alter() would seem to be the one you are looking for to change execution order.

However, I'm not sure It is exactly what you are looking for because it could have to do with the node edit form. One field could be higher on that form, and that could be causing the validation order issue. The weight of the fields on the form itself could be dictating the validation order. If that is the case, you can hook into the node form and run the validation in the order needed.