I know it's 4 years later, but I went back to the game and realized what I was missing. I thought that the game will give you a tile that is definitely going to fit up until the board is over. But, of course, the game gives you a tile randomly, and if it no longer fits, he game ends.
But because that last tile is not shown, during the end game you never really know when the game will end once you have limited options. You can get lucky and continue receiving tiles that fit existing shape, or you can randomly get a shape that cannot fit anywhere, and the game ends, although there are potential tiles that would fit.