Outliers in Google SERP are results that do not seem to match the intent of other web pages in the search engines’ result page.
An outlier is an observation that lies an abnormal distance from other values in a random sample from a population.
When a search is performed Google has a topic lifecycle to try and learn what users want to see from previous behavioural signals.
If the behavioural signals are showing mixed results then the first page will show various sets of intents from positive sentiment, negative sentiment, information posts to listicle articles.