Bunker mentality is a slang

Slang

Slang is the use of informal words and expressions that are not considered standard in the speaker's language or dialect but are considered more acceptable when used socially. Slang is often to be found in areas of the lexicon that refer to things considered taboo...

 phrase for a phenomenon that occurs when an individual or group stops taking into account new, pertinent information and begins viewing outsiders as enemies due to an isolation resulting from being under attack. Political campaigns and figures are often accused of having this mentality, particularly when a leader, administration or party has become unpopular or is in some sort of trouble.

The phrase is taken from an analogy to soldiers that have taken shelter in a bunker

Bunker

A military bunker is a hardened shelter, often buried partly or fully underground, designed to protect the inhabitants from falling bombs or other attacks...

 while under siege from enemies as well as Adolf Hitler

Adolf Hitler

Adolf Hitler was an Austrian-born German politician and the leader of the National Socialist German Workers Party , commonly referred to as the Nazi Party). He was Chancellor of Germany from 1933 to 1945, and head of state from 1934 to 1945...

's mental state during his last days inside the Chancellery bunker in Berlin.