They do do this! When you're on a question, for example https://stackoverflow.com/questions/30081275 https://stackoverflow.com/questions/30081275, click on the

They do do this! When you're on a question, for example https://stackoverflow.com/questions/30081275 https://stackoverflow.com/questions/30081275, click on the answer sort order drop-down and select trending. I've also seen the trending sort automatically applied on certain questions.