Why did Sauron fear Aragons coronation?

[Inziladun]

Originally Posted by Zigûr

That being said it seems to me that Aragorn's activities as Chieftain of the Northern Dúnedain, as Thorongil in Rohan and Gondor and his adventures in Rhûn and Harad must not have gone unnoticed even if Sauron wasn't aware of his lineage.

A line from the Unfinished Tales section The Hunt For the Ring could contradict the idea of Sauron being interested in Aragorn personally.

It would then no doubt be in April before Sauron heard that Gollum had been seen again, apparently captive in the hands of a Man. This might mean little. Neither Sauron nor any of his servants yet knew of Aragorn or who he was.

That would seem to indicate that Sauron and his servants were simply vigilant for signs of an Heir of Isildur's return, though as Aragorn noted after his partial reveal to Sauron, the latter probably in his heart believed it wouldn't happen.