lab attacks chihuahua

Question
My neighbor's female chocolate lab attacked and killed my mother's chihuahua tonight.  They have lived across the street from each other for six years.  The attack happened when I had the chihuahua on a leash.  The lab and a poodle were smelling the chihuahua for about five minutes as I spoke with the owner.  The poodle started playfully nipping at the chihuahua, then the lab started biting.  I pulled the chihuahua up by the leash and ran away with him, but it was too late.  What on earth could've caused this?  

Answer
I am sorry to hear that.  It is hard to say just what happened.  Labs usually manage to play just fine with smaller dogs and puppies.  Dogs still think like wolves.  Their instincts have not caught up to the disparities in size man has bred.  I hate to blame the victim, but it is possible the Chihuahua gave the Lab a challenge he couldn't back up.  There are some subtle body language  and head position things that mean trouble.  I have often seen smaller dogs have to be rescued after they challenge bigger ones.  Unfortunately those things can happen in a flash before you realize there is a problem.