I've yet to see a slitter or hollow-tine grass aerator that would penetrate to the depth reported of 20". Grass is essentially shallow-rooted to perhaps 2" and such depths would be unnecessary. Most likely is the cable, reportedly laid 20 years ago, simply failed from a combination of rusty armoring, age and the forces associated with frost-heave. Southern England was uncharacteristically cold this winter [ to 3F ].
How does a four-legged animal get a shock - isn't grounding supposed to make things safe?