a spontaneous interview of a politician, usually after closed-door meetings (see Image 1).
2 v. — Politics
to crowd around a politician in order to interview him or her.
Type: 3. Semantic Change — Not long after the documentation of the noun, a corresponding verb was created (see the 1981 quotation for the earliest quotation thus far). The term is Canadian, both in meanings 1 and 2, by virtue of frequency (see Chart 1) and as a culturally significant feature of Canadian politics and its particular traditions.
See also ITP Nelson, which lists the term, s.v. "scrum (2)". but does not label is as Canadian.
Images:

Image 1: A scrum with Justin Trudeau. Source: Radio Canada International. Photo: J. Tang 
Chart 1: Internet Domain Search, 12 Oct. 2012