This question answered my question "why some questions has yellow background" but it raised another question. I am viewing questions under my interesting tag then what is the need for yellow background because all questions fall under my interesting tag.
According to this answer, if you're viewing questions by tag, then the question should only be highlighted if they contain another interesting tag as well.
Although you are viewing questions that you are interested in (or tags that you are active in) there are questions in there that you aren't interested in.
If you are logged in you will see 9 questions (18 for anonymous users) that are picked by random from the last 3000 questions.
Hence, why most of them are highlighted but 9 of them won't be as you haven't marked the tag as interesting.