10:10 Jan 9 

The reason is that the pronunciation of -ll- and of -y- is very similar, and many speakers of Spanish pronounce them exactly the same way, though they should really be distinguished.

What happens is that some speakers reduce /ll/ to /y/ in speech, but are aware that words with a /y/ sound are often spelled with -ll-, so they hypercorrect words like "hayan" which really should be spelled with a -y-.

10:06 Jan 9 

Oh, well, that makes things rather easier .... I wonder why it's a common spelling mistake?

10:00 Jan 9 

There is a common spelling mistake in your source text: "hallan" should be "hayan".