Could my dentist be the cause for two root canals: Excessive grinding in preparation for crowns for a temp 4 tooth bridge?

It is possible to over prepare teeth but it is more likely that there was decay present initially or that the teeth decayed or were subject to bacterial leakage during the long time a temporary bridge was in.  Usually a temporary or provisional bridge is in for a few weeks and then a permanent one is placed.  You did not say why four teeth needed to be extracted.  The normal reasons are severe disease , either decay or periodontal disease which probably also affected the adjacent teeth.The dentist is probably not responsible either causally or financially. 

Yes it is possible that preparation of the teeth is what caused the nerves to become irreversibly traumatized. However, it does not mean that it was excessive or avoidable. The amount of grinding likely was completely necessary in order to remove old filling or decay or to provide enough space for the bridge material to be made strong enough and still allow you to bite normally. It is unfortunate that the trauma of treatment can result in needing root canals but it does not mean the treatment itself was inappropriate.

Yes, of course, the more the teeth are prepared the more likely root canals would be necessary.  Also teeth that have had large fillings or cavities, or trauma are also at high risk.That being said, in many cases the teeth are are misaligned to the point that bridges cannot be done without more than normal amount of tooth preparation.  Unfortunately, this is a common complication that dentists and patients have to deal with and be able to accept.Whether or not your dentist did "excessive" tooth preparation could not be determined without seeing models of your teeth prior to the preparation, models after the tooth preparation, x-rays of your teeth and a dental examination.  It is not that easy to just blame your dentist for you needing to have root canals.The main point here really is to get you out of pain first, and make sure you have a well made bridge.  If you distrust your dentist, then by all means, seek the opinion of another dentist.