Apparently, the courtesy of announcing his arrival before stepping inside her home was something that Aberforth Dumbledore wasn't prepared to offer. Hermione knew enough about how to effectively interact with one's enemies to understand what he was doing. From the very start, he wanted her to understand that he held the power. It was an iron-fisted enough action to make her smile despite her dislike of the man. He might be there to teach her how to cloak her magic, but he wasn't going to be too friendly. Just what she appreciated.