Drilling a hold at the end of the crack changes the Stress Concentration Factor (SCF) at the crack tip. There are both ductile (iron, steel) and brittle (glass) cracking. The stress at the crack it is theoretically infinite. In a ductile situation, the area around the crack yields reducing the stress and the energy. Cycling through stress ranges slowly propagates the crack as it yields. By drilling a hole, you change the stress distribution and the imperfection size thus eliminating the ability for the crack to propagate.
In order for a new crack to start on the other side of the hole you need an imperfection. Drilling provides and almost perfect surface that resist cracking.
The only other way for the crack to form is from the welding stresses AND imperfections. The preheat and slow cool down eliminate/minimize most of these stresses. In industry Non-Destructive Testing (NDT) (X-ray, magniflux, etc) main purpose is to minimize the imperfections to a size that is small enough that cracking wont be an issue during the design life.