What Astarion tells you is speculation on his side, though.
Astarion's speculation is based on his knowledge of the parties involved, which is pretty extensive, and everything he's told us about them is consistent with his explanation. Note that he's not surprised
to find Cazador sending hunters after him, or that Cazador would choose to send the Gur as an extra twist of the knife. He's only surprised that the orders are to bring him back alive - which, more than likely, is because Cazador put a Very Important Thing on Astarion's back (which we're gradually learning more information about) and Cazador wants it returned intact. There's no reason for the Gur to want him alive if all they're after is revenge on Astarion. Have their hunter finish the job they started two hundred years ago and move on. Simple.
In any case, my original point was that the party is about ten days' journey from Baldur's Gate, yet it's only a few days after the crash that someone from the city arrives on site. There's no time for news to have traveled so far and for someone to have followed up on it by normal means. Is it a plot hole, or did the abductions happen quite some time ago - long enough for an extensive network of agents to be deployed - or
(and this is how I'd write it, if I were writing it) has it only been a few days since the abductions, but Cazador has enough power/influence to employ a scrier to track Astarion as he's carried aboard the nautiloid, and once it crashes, to see that hunters are teleported to its vicinity.
Maybe the gur leader wants Astarion as leverage against Cazador. I heard Cazador can't actually move around much for some magical reason, but keeps quite the deadly army.
That would be interesting. Apparently there are factions of some sort involved in the future story,
though I've only seen mention of undead factions, not Gur. Presumably there are people who want Cazador out of the way because he's a pesky evil vampire lord preying on their city; gathering a group of them to take him out would be fun. I'm not sure how well Astarion would work as a bargaining chip. He does mention that Cazador never leaves the city, but I think that's because he needs to go back to his coffin during the day, as all proper vampires must.
