Learn Hardcore Hungarian: Arra hivatkozva, hogy viharkár volt, kárigényt nyújtunk be. - Citing the fact that it was storm damage, we submit an insurance claim.

Arra hivatkozva, hogy viharkár volt = on the grounds that there was storm damage
The whole sentence is an official-sounding way to say that a claim is being submitted because storm damage occurred.

Why is arra there? What exactly is it doing?

Arra is the pronoun az in the -ra/-re case.

It is there because hivatkozni normally takes this case:

When the something is not a noun but a whole clause, Hungarian often uses this pattern:

So:

This is a very common Hungarian structure:

What is hivatkozva? Is it a normal verb form?