It’s has allegedly been growing for the last few months.
Apparently the disagreements between Bolton and Trump, and those in the highest levels of the White House, have become increasingly worse, and the administration is unhappy with him.
Bolton, a staunch proponent of continuing with the active military campaign (war) in Afghanistan, really is in contrast to how Trump has always said he seeks to end that war.
I’ll update when links become available.
However, just now (12:16pm ET) on FNC Bolton texted Brian Kilmeade and said that he “resigned”, and was not fired.