니까 끝나다

…and then it was over

"-(으)니까" here has two jobs and both show up. As "on doing it, it turned out": we were having such a good chat that it was over already, doing this and that it just ended, I closed my eyes and opened them and it was done. As a plain reason: I don't know what this is, so tell me afterwards. 끝나다 is the thing coming to an end by itself, not someone ending it — that would be 끝내다.