why is it cuya in line 1 instead of cuyos? surely it refers to the incendios, masculine plural?

Hola Rosemary

If you have a look in the lesson for cuyo/cuya, cuyos, cuyas, you'll notice that we say that the agreement is with the possession, not the possessor, so in this case it is agreeing with "causa", not with incendios. It agrees with what comes after "cuyo,-a,-os, -as". 

I hope it clarifies it for you. 

Saludos

Inma