Several reasons nocks can readily break. Two common causes are 1) too tight on serving, or 2) serving (string) not full seating in nock.

Another cause often overlooked is that when the shafts were fletched the chemical used for cleaning shafts got on or to near nocks. Also, when the shaft is point down while fletch is drying, the vapor from the glue (especially super-glue type) and the cleaner can rise and engulf the nock. Any of these chemicals do have the capability of affecting the integrity of the nock and causing the nock to weaken or become brittle.