Change in behavior

Question
We have had 2 different friends that have had yellow labs.  Both seemed to be excellent family dogs but at one point turned on - one a small child and bit her and the 2nd turned on both owners and bit them.
They both ended up putting their dogs down. What causes a change in dogs that have been trained and loved by a family.  They both decided they couldn't have a dog that did that.  I feel bad for both of them. The are considered a great breed for a family.

Answer
I would say that it all depends on where they got the dogs.  Dogs purchased from breeders who are reputable and put alot in to making sure temperament is KEY, they typically do not have problems.  

Sometimes there are underlying health issues that cause a dog to snap.  

Also "well trained" is subjective.  A dog may be easy to live with and obedient, but not necessarily well trained.  And if the dog did not learn that the people are in charge, and that includes the kids, and the folks allowed the kids to mess with the dogs when they were sleeping and eating, even a well trained dog can snap.