I have talked to several people who have Jack Russell terriers
who have had them bite for no reason. Terriers
can be notoriously difficult to deal with anyway, but the problem with JRTs is that they have become VERY overbred in the last 10-15 years, and where extra care should be taken with regard to temperament in ANY breed, it is even more important when breeding terriers.
If your dog has never before shown any signs of aggression, the first thing you need to check for is a source of pain.