A Match Has Been Found

While I was in champ select, there was a popup titled "A Match Has Been Found" There were two buttons. One said "Finish What I was Doing" and the other said "Play Now".
I clicked "Play Now" and client went black. I waited for a while with nothing happening. Chat still worked though. After I thought I had waited long enough, I decided to restart client and then I got a dodge penalty.

What was the reason for that popup?