One of the "best" speculation/analogy I read was that the New Republic didn't want open war with the First Order, so Leia goes and forms the Resistance.... but the New Republic gives them weapons and equipment. Much like the USA would give weapons to "resistance" fighters in various countries they didn't want to have open war with but would like it if someone else was in charge of.
So then the First Order, despite their main combative enemies being the Resistance, are sour at the New Republic and blow them up.