What decides who gets to fight in a succession war?

[John Fish]

I have here two examples where successions didn't make sense.

1: I was playing as Scandinavia, and Portugal claimed the throne on Castille. After their ruler died, I was given the option to start a succession war with Portugal for the throne of Castille. I don't actually know why because as far as I remember I didn't have a royal marriage or the same Dynasty with castille, and I remember we were enemies before. Seemed kinda random.

2: A succession war didn't actually start here, but playing as Byzantium I had a royal marriage with Austria, and the same Dynasty as Hungary (Palaiologos). When I had no legal heir, a notice appeared saying that if my ruler were to die, a succession war would start between Austria, and France of all nations. France hated me with a passion, and I didn't have a RM or the same dynasty. Why would a war start between France and Austria and not, for example, between Hungary and Austria?

What decides the second contestor for the throne?

[Bishop]

from EU4 wiki:
The result of claiming the throne depends on whether or not the current ruler dies before producing an heir, and on your prestige. If you or the target country with no heir has rivals with higher prestige, they can start succession war.

[Kagemin]

I think there's a pretty recent guide on the mechanics, I remember reading about it on the forum.
Edit:
http://steamcommunity.com/sharedfiles/filedetails/?id=293290169

[Fat Ungrateful Squirrel]

You can view the guide, but short term -
A succession war will start with said country and the said countries most prestigous rival
Example: France will get a union with Hungary, Austria, their most prestigous rival can claim the Hungarian throne in name.
Also, being at war makes it impossible to get inherited/pu'd if your ruler dies, it replaces your normal Union over nation to a noble.