due to several factors
First - they can be Electrocuted by them due to DC Current running thru them or be entrapped in the wiring .
Secondly- they pose a major risk of roof collapse during a fire due to there weight they put on the roof
Thirdly - a roof fire can not be gotten to under the panels allowing the fire to spread.
Which is what happened at a massive warehouse blaze at Dietz & Watson in Delanco NJ
The danger solar panels pose is so great that some states now require a very clearly marked plaque
be placed on the outside front of homes and business warning firefighters of the danger and systems be registered with local officials .