Root Canal + Veneer?! What's going on?!

This is not a simple question to answer. Photos and x-rays certainly would be helpful. Your open bite well could have been caused by trauma to the lower jaw which prevented it from fully forming. When this occurs an open bite develops. The trauma well could have damaged the pulps of some of the front teeth without any significant discomfort. Now if there is an abscess the teeth will need root canals. Your dentist well might be identifying a problem that has been brewing in the bone for years or even decades. Good luck.