Fushin (普請) is from Buddhist terminology and the reading "fushin" was introduced through Zen Buddhism, therefore To-on (Japanese reading of kanji [Chinese characters] imported from China by Zen priests) (唐音) is used (See Dipankara and similar works).
普請は仏教用語で、ふしんの読み仮名は禅宗が伝えた為、唐音が使われる（燃燈仏などを参照）。