Is the word “恶犬” trending IRL?
If you want to track the trend of a word in real world, especially when this word may not be popular in a large area or on the Internet, you should do it yourself. Ask local residents living around, or these shop keepers who decided to use this word to find out the reason behind. Maybe looking up local newspaper also helps.

“恶犬” (fierce dog) is not a new word, I think it's mostly used around factories or private properties to warn irrelevant people to keep away. It’s seen in rural areas more often. I hardly saw it used by pet shops (and I think they should use "lovely dog" instead). Interesting if it can be confirmed as a new trend.