Quite frankly, there is no way of telling why your dog only growls at Ugg boots. If she is adopted, then it might be that the Ugg boots remind her of unpleasant treatment at the hands of someone who used to wear boots; dogs do learn and remember.
If they are lined with real sheep fleece your dog may pick up the scent. Or maybe you picked up another animals scent on your boots. Either way your dig does not like them. Your pet may start chewing on them . if you like them put them away.