Since it happened while I was playing near the top, I had a feeling it was similar to one I had that happened as a result of having selected and trying to match a fruit as it was getting eliminated as waste. Oh, and it doesn't get locked, but rather displaced by one on Y (you can keep playing "normally", except you select the tile above the cursor).
The only way I managed to reproduce it, was by leaving a lot of single tiles, selecting them and trying to match them as they were getting eliminated. It's hard to pinpoint the exact trigger conditions because you only realized it happened afterwards, when you're trying to select another tile.
It could be unrelated, but when it happened, a weird glyph appeared on the last digit of the score (went back to normal as I kept playing).