Electricity flowing between a generator and a customer moves through all lines connecting the two, not only along the shortest distance between the two points. Grid operators must carefully balance power supplied to the system with power being used by customers so that individual lines are not overloaded.
Neighboring transmission systems are usually connected together, and this can complicate power flows even more.
In the example above, the North and South Grids are interconnected at two points. Electricity dispatched from the North generator to its customer can thus flow into the South grid. In certain situations the electricity might flow into and back out of the South grid -- a "Loop Flow."
Loop Flows are difficult to control and can damage transmission equipment. Sometimes, the only way for a grid operator to prevent damage is to declare an emergency and cut power flows through the interconnection.