Thanks to everyone for the replies.
Just to be clear, I have complete faith in this mechanic. He's one of the most competent (been in business for over 30 years and is sought after by car-owners from across North America to work on their high-end, and expensive, classic autos) and honest mechanics I have ever met (that, and he showed me the mount). So, I trust what he is telling me. And, the mount is only cracked at this point and not yet separated (he tells me I can still drive it until it is taken in for repair on Monday).
Was just curious whether anyone knew if this is fairly common and if it could be related to towing a trailer.
Thanks again,
Marv