Here is an interesting video from CNBC's "Tech Check" series. Manufacturers of solar panels pay Underwriters Laboratories to do extensive testing of their products. The panels are tested for a period of 75 days and undergo tests such as having simulated hail shot at them, exposed to intense heat, slammed with a heavy object, etc. The panels that pass are given the OK by Underwriters and given the UL listed and approved tag.
According to Jay Holman of Clean Energy Blog the risk of rooftop fire is real when talking about rooftop solar installations. Rooftop fires are not a common occurrence but he says a potential problem is caused by an "arc-fault" when electricity arcs across an unwanted air gap in a circuit.
The older the panels the more of a concern - especially poorly made panels that are aging and deteriorating. It will take years before we see if the current solar installations are going to cause problems in this regard. Always helpful to be aware of potential problems.
Here's more about Arc-Fault Circuit Interrupters.
Clean Energy Blog.