Japanese text is commonly written either left-to-right or top-to-bottom. You'll see top-to-bottom most often in newsprint or manga.

After clicking on the little 'person' icon in the lower right of the control you're taken to the author's personal site I assume. The site seems to indicate that they're either located in Japan or natively Japanese.

In this case the Japanese text is correctly oriented, however the English text is rotated 90 degrees as a result. Since they author is probably aiming for a Japanese audience this doesn't come off as odd.