The electrical grid is literally connected to ground at points that can be separated by hundreds of miles. Ground currents cause those points to be at different voltages. Since the wire is very low resistance, this can cause very bad damage if the protection fails.
> There may be a correlation between the currents and weather/natural events.
Ground currents are caused by the same thing as Auroras, in addition to normal weather. They occur constantly to a small degree. They are a problem everywhere in the world; if you lay out a 100 mile metal wire without taking proper precautions to make it safe electrically it will quickly start creating very dangerous discharges.
Think about it this way. It's very dangerous to fly a kite in a thunderstorm; the line will conduct electricity and makes it much more likely that the massive difference in charge between the air and ground will pick you to flow through. Ground currents are the same situation, except between two bits of ground. Civilized areas of the planet are totally cobwebbed by kite lines connecting these areas. It would be bad.
> The James Bay network went offline in less than 90 seconds, giving Quebec its second massive power outage in 11 months.[10] The power failure lasted nine hours and forced the company to implement various mitigation strategies, including raising the trip level, installing series compensation on ultra high voltage lines and upgrading various monitoring and operational procedures. Other utilities in North America and Northern Europe and elsewhere implemented programs to reduce the risks associated with geomagnetically induced currents (GICs).[9]
https://en.wikipedia.org/wiki/March_1989_geomagnetic_storm
