Certainly some of these cases where dogs are shot are the case of some minor or major phobia of the specific officer involved, but there's no way anyone could ever convince me that's the majority of cases for how often this happens. As an aside, if you have a phobia of dogs, you should not be allowed to be an officer. There are far too many situations where encounters with dogs will happen to allow someone who has a panic attack and loses control of his emotions and gets an itchy trigger finger to be trusted with such a responsibility. So, if we assume that most of these cases are not instances involving officers with this very specific phobia, there can only be three explanations:
1) The officer does not have an unreasonable fear of dogs, but genuinely believes himself or others to be in significant danger and that lethal measures must be taken. In some cases, the officer is correct, and a large dog(particularly ones that have been trained to fight/guard) can be a real threat. However, much of the time, this does not seem to be the case. We have countless instances of small to medium sized dogs who pose little to no threat to a full grown man/woman, yet get gunned down still. We have tons of instances where large dogs that are not being particularly aggressive are shot as well. So, unless there is actual merit to the officer's claim of being in danger(which we must determine without taking the LEO's word for it, he/she is not going to condemn him/herself), then this explanation is not nearly as justified for how often it is used.
2) The officer is sadistic/hates animals and wishes to inflict emotional harm on the owners of the dog. I find this to not be very likely in almost all cases, though there are sickos in every line of work.
3) They don't care and they don't fear the repercussions. The officer simply feels like he or she can get away with it. They may not particularly like or dislike dogs, but they certainly don't care enough to value someone else's pet. It happens so often and with little consequence that the "it's coming right at us!" farce is sufficient to answer all questions. This is what it boils down to, I think. Lack of empathy for civilians, and no fear of being reprimanded for killing an animal. They simply feel like the risk of actually being attacked by the dog is not worth finding out if the dog is indeed aggressive and a legitimate threat, so they kill it prematurely, not worrying about anything else but themselves.
and for fun, a 4th option.
4) They're scared little ******* who shouldn't be officers to begin with.