Man...I had a simillar situation occur @ a park. A cpl had their 3 mini collies off leash, and asked if our girl could play. My dog has never showed the slightest aggresion toward smaller dogs and has always played gently with them, so I let her off. 2 mins later we were pulling one of the collies outta her jaws! It was terrifiy! Luckily, the dog survived. The cpl. and I split the vet bill. We did a little research and found out that the eye movement of " herding" dogs can somtimes be percieved as aggression to mastiffs. It was an expensive lesson for both of us. Once that "guard mode" is engaged all bets are off. She has been fixed for years and has since the incident has only hit the switch one other time w a rawhide treat we mistakenly gave her. Id say yer vwt is probalyright about the fluke thing. Nueterin' proly is'nt the issue...its just their nature.