Great question, let me help you.
This is called External Resorption
It is called an idiopathic condition, which means nobody knows the reason teeth do this
This is a rare condition, so there is not much research on it
Unfortunately, there is nothing you can do to stop External Resorption b/c it is your body attacking itself (the tooth).
This tooth will eventually have to be removed
There is no evidence that since one tooth has it, others will too
So it is unlikely that other teeth will follow suit
I'm sorry you are going through this, and I wish I had a "silver bullet" cure, but unfortunately it is a rare condition that cannot be stopped
Thanks in advance for clicking accept