Why did my teeth die? Invisalign and Acceledent too aggressive? Can local anesthetic kill teeth?

I've been wearing Invisalign for over a year. Recently I found out I have a dead tooth. My dentist tells me all my front teeth look like they could be dying. I've never had trauma on them. He concludes it's the braces. I confront my orthodontist and he denies it is the cause. During my Invisalign treatment, I had a filling done on that now dead tooth. Can local anesthetic kill teeth? How about electric toothbrush + Acceledent = overexcited nerves? Was my orthodontist too aggressive?