The ground electrode corrodes (unnoticed as they're rarely tested), increases the ground resistance, and then the voltage on the lightning conductor itself is high enough to arc off onto other items, as happened in this case. Very common failure with older installations.
I have also seen a church damaged in exactly the same way. Didn't do as much masonary damage, but arced through to the wiring in the tower, and then almost everywhere where a piece of T&E was clipped around a corner, there was about 12" of it missing either side of where the bend had been.
The lightning conductor earth rod could be pulled out of the ground afterwards, and there was only about 12" of it still connected to the copper strip - it had corroded through (although the lightning may have burned though the last bit).