Answer: Fear and anxiety created by veterinarian visits.
Help is on the way. The Fear Free initiative developed by “America’s Veterinarian,” Dr. Marty Becker, aims to “take the ‘pet’ out of ‘petrified’” and get pets back for veterinary visits by promoting considerate approaches and gentle control techniques used in calming environments. Soon Fear Free certified clinics, developed in cooperation with the American Animal Hospital Association, will aim to eliminate fear, anxiety and stress and create an experience that is better and safer for all involved, including pets, owners and veterinary teams.
Source: http://www.fearfreepets.com/fear_free/default.aspx