Why did Sauron fear Aragons coronation?

[Inziladun]

One thing I find curious that may bear on this is Aragorn's statement to Frodo on their meeting at Bree.

'I did not intend to tell you all about myself at once. I had to study you first, and make sure of you. The Enemy has set traps for me before now.'

That suggests that Sauron was worried enough about the idea that an Heir of Isildur remained to have in some way been looking for evidence of such a person. Aragorn, of course, does not elaborate on the "traps", but it says something that even before the War of the Ring the Heir of Isildur was on Sauron's mind.