Yes, I've had the same thing happen - string broke while the bow was hanging on the rack. Veery lucky it wasn't earlier that day, at full draw.
There have been a couple of threads here on this.
A nock tied too tightly can compromise the serving, especially if the diameter of the nock material is not thicker than the serving material. Glue can also be a cause.
When using a tied nock now, I'm careful to use thicker material, and keep a close eye on the serving. Also I melt the tag ends into the tied nock, and avoid glue.