My thoughts is that after the battle of Azanulbizar, Azog, who had already sworn to wipe out the line of Durin, would have been doubly so after losing his hand to Thorin-after he stuck that claw in the wound and healed of course. And he knew that Thorin survived the battle. The dwarves had likely (not sure about timeline but PJ shortened it from the book) settled in Ered Luin (blue mountains) right after the battle or at some point not too far away from it I believe, and it would have been very difficult for orcs to make it all the way west and so far away from their master to storm Ered Luin. So I would imagine that Azog had been hunting Thorin for years, and must not have gotten lucky with Thorin being so far west.
As to specifically setting out to hunt him at the time of the quest, I think IMO that the orcs were there possibly continuing a decades long hunt for Thorin and that when Thorin finally after so many years out west ended up east where the orcs happened to be, they finally found them and began officially hunting (meaning following tracks and such) after that night on the cliff when they were spotted. Why the orc pack crossed the Bruinen is probably the same reason why the trolls came down from the mountains-not necessarily to specifically hunt Thorin, but rather because Sauron's presence was growing again and foul creatures were once again gaining the ability to start venturing further and further from their strongholds.
It also could be that Thorin and Gandalf had a spy amongst them when they talked about the quest at Bree before the company was even gathered, and the spy informed Azog that Thorin would be heading east soon-closer to Azog and orc/goblin territory (the Misty Mountains). It is perfectly plausible-they met in the Prancing Pony, and when Frodo and Sam and Merry and Pippin were there, the "squint eyed southerner" from the book (not sure if he was ever named) was discovered by Aragorn later on to have been spying on them along with Bill Ferny while they were there. So it is possible that someone of a less than savory nature eavesdropped on their conversation and somehow got the message to the orcs that "hey, you know that guy that you've been looking for for decades? He's about to go waltzing straight toward you, so hold off the invasion of a mountain hall but rather let him walk to you and attack his teensy company in the middle of nowhere where you are far away from the rangers and men who would kill orcs like you"
I have a feeling that from Bree on to the west it was far too populous (plus the servants of Sauron didn't appreciate water all that much and there were also too many rivers in the way) for the orcs to travel across to get Thorin-they would have died very quickly long before ever reaching the Blue Mountains.