The issue is most likely caused by the roof-mounted micro-inverters for each panel/zone and associated wiring. All it takes is a loose connection. Just like a shore power cable. Micro-inverters are more efficient but also more failure prone. For instance, with 100 panels and 100 microinverters your MTBF (mean time between failures) just got divided by 100.
However, shorts in solar panels themselves can happen too.
We have a 10kW/ 40 panel array operating continuously for the last 12 years. It is ground mounted, so easier to install/inspect/maintain. Out of 40 panels with 60 solar cells each, I noticed one anomaly recently. A single cell seemed to short internally, producing heat and discoloration. The local cell temperature at the short was 205 deg F, the rest of the panels were at 110 deg F (on a 75 deg F day). On a hot day at noon, temperatures would be significantly higher.
There is not enough current flowing for a single cell short to cause a fire, but a multi-cell short definitely could. Good idea to keep your panels in a large high-voltage array away from combustible materials.
