Written via a giant calligraphy brush (shodō (書道) on washi paper, the priest wrote the kanji 1.5 meters in length x 1.3 meters in height.
Apparently, the voted upon kanji was felt to describe the number of man-made and natural disasters that seemed to afflict the country.
This year - 2018 - there was heavy rain and mudslides in the west, and a big earthquake in Hokkaido, plus there were man-made disasters (sort of) involving stolen cryptocurrency and coaches harassing athletes.
I'm unsure how the later two are "disasters" on the scale of the kanji... if you look at the kanji, 災, it is quite obviously a pictogram representation of an active volcano. Hardly man-made.
As mentioned, the kanji is voted upon by the Japanese populace, with 193,214 entries received this year, with sai raking first with 20,858 tallies, according to the Japan Kanji Aptitude Testing Foundation.
The kanji of the year 2017 was kita/north (北), as Japan was rightly concerned with North Korea and its missile testing over Japan.