go (in the sense "become") + postal; from a rash of incidents, mostly gun violence, perpetrated by disgruntled U.S. Postal Service workers on co-workers in the United States in the 1980s (see quotations)
go postal (third-person singular present tense goes postal, present participle going postal, simple past went postal, past participle gone postal)
#intransitive US informal To behave in a hysterical, angry or irrational manner.
trans-top|behave in a hysterical, angry or irrational manner
German: ausrasten informal
1993 The symposium was sponsored by the U.S. Postal Service, which has seen so many outbursts that in some circles excessive stress is known as "going postal." Thirty-five people have been killed in 11 post office shootings since 1983. — Karl Vick, "Violence at work tied to loss of esteem," St. Petersburg Times, December 17