Any theories why it would hit the water line?
I would imagine that at least 50% of Discharges will involve high levels of Current flowing via Underground City Water Lines.
The Water Lines assist with the transfer of large amounts of Majority Charge Carriers - either prior to &/or during
the "Connection" of Stroke Leader(s), and the complete Discharge event.
Prior to the "Big Event" (where the Stroke Leaders connect and initiate an Arc - the predecessor to the large
discharge path Plasma), there will be Hundreds of Non-Visible "Leaders" rising up from the Ground level (typically Trees and tall Buildings, but also from the actual Ground its self).
At the same time, there will be Hundreds of Leaders extending outward from the Cloud Mass above.
As the Charge in the Cloud Mass increases (gains more Negative Charges), an equally increasing Charge is impressed on the Ground directly below the Cloud Mass.
The large Negative Charge State of the Cloud Mass "Pushes" Negative Charges in the Ground below it "away", resulting in the Ground in that area becoming high in Positive Charges.
This continues until the Breakdown of the Air is reached (apx. 10KV per Meter), and Two (or more) Stroke Leaders connect between the Cloud and the Ground.
Discharge occurs until both the Cloud Mass above, and the underlying Ground Area have once again reached a near equal Potential.
Typically, this is when the Cloud has discharged all the "Excessive Negative Charges" which were recently accumulated.
The Underground Water Lines help transfer Charges very efficiently, acting as a highly conductive path to transfer Positive and Negative Charges, to & from the discharge event location.