The Diplomacy 1.2 version is the most "level" when it comes to pirate bounties and actions. Here's what the AI does seem to tend to do though:
If there is an equal chance with no clear advantage of the pirates attacking any player, there's a slightly better than average chance it will be you.
If the AI decides to meet and raise your bounty to get the pirates to head your way, the decisive bribe will happen 1/10th of a second before the raiders launch.
When pirates do attack you, it is almost a certainty that any AI "players" near you with no other groups between them and yourself and who are close to or superior in strength to youwill attack you at pretty much exactly the same time as the pirate raid.
Aside from that, it's pretty random and balanced now.