Question

what caused the red scare/ mcCarthism?

Answer 1

The detonation by the USSR of an atomic bomb in 1949, about 10-20 years earlier than anyone thought possible, and the revelation (via the secret Venona transcripts, only revealed many decades later) that this had been enabled by Soviet atomic espionage.

In short, the fear of nuclear annihilation by Stalin's Soviet Union, coupled to the fear that the disloyalty of Americans that had enabled Stalin's quick possession of the bomb could lead to further compromises of national security.