The owner's comments don't make a whole lot of sense. If the only problem the dog had was barking, and that was enabled because the collar that shocks him when he barks' battery died, it also presumably would not prohibit him from exiting said electric fence. The dog, according to the article, was used as a bear-dog, which are trained to aggressively confront animals substantially larger than themselves in the hopes that they run off.  

So, we have a dog, that by the owners own statements was barking yet again in this situation, which would lead a reasonable person to suspect that the batteries were once again dead in his collar, and was, a such, unrestrained again. We also the pesky fact that the dog was over the property line, which indicates the dog obviously also approached the man, refuting a construct where the dog was just sitting in the middle of the yard minding his own business. The guy had no reason to believe the dog was harmless, and in all likelihood was in fear.