apropos of nothing; to change the subject
Used as a conversational opener when suddenly changing the topic or bringing up something unrelated. Often softens the abruptness of the new subject.
This is out of the blue, but I'm moving tomorrow.
Sorry to change the subject, but there's something I want to ask you.
A fixed phrase combining 突然 (suddenly) with the explanatory/softening だけど (but, though). The exact origin as a set expression is uncertain.