The variation of pronouns in the Qur’an is a sign of its eloquence and miraculous nature

The Qur’an
was revealed in a plain Arabic tongue; one aspect of the literary style of
the Arabs is that the speaker may refer to himself sometimes in the first
person, sometimes in the third person, sometimes in the singular and
sometimes in the plural. This variation is part of eloquence and good style.
No one can understand this except those who know Arabic and have a
sufficient grasp of its different ways of expression. 

The Qur’an
was not revealed in only one style, as the questioner thinks; rather it uses
a variety of different styles; this is part of its miraculous nature and
eloquence. 

Dr ‘Abd
al-Muhsin al-Mutayri says in his book Da‘aawa al-Taa‘ineena fi’l-Qur’an
al-Kareem (p. 304): One of the literary styles of the Arabs is for the
speaker to refer to himself in the first person and in the third person. For
example, a speaker may say, “I did such and such; I went; I instruct you, O
So and so, to do such and such.” And sometimes he may also say of himself
that “So and so – meaning himself – instructs you to do such and such, and
forbids you to do such and such; or he likes you to do such and such.” This
is like when an ameer (ruler) or king says to his people: “The Ameer asks
you to do such and such.” What he is trying to emphasise is that his
instructions are based on the fact that he is a ruler or king; this is more
eloquent than saying to them, “I am the king and I am telling you to do such
and such.” Saying “The king instructs you…” is more eloquent than saying “I
am the king and I am instructing you…” 

This kind
of style also appears in the Qur’an. The one who does not know Arabic may
think that Allah cannot speak of Himself in the third person, and that He
must say “I have sent down to you, O Muhammad, the Book with truth,
confirming what came before it” and the like. But this reflects ignorance of
the literary style in Arabic and how it is part of Arabic eloquence.
Undoubtedly for Allah to speak of Himself in the third person is more
eloquent than saying “Alif-Laam-Meem, I am Allah, there is no god but I, the
the Ever Living, the One Who sustains and protects all that exists. I sent
down to you the Book with truth, confirming what came before it…” (cf. Aal
‘Imraan 3:1-3).

End
quote. 

It is
known from Arab literary custom that they do not persist in one style in
their speech; rather they move from one style to another, even in a single
passage, let alone when there are two different passages (on two different
occasions). This is part of Arabic literary style that serves to keep the
listener’s or reader’s attention. 

Az-Zarkashi (may Allah have mercy on him) said: 

Moving
from one style to another serves to make speech flow more smoothly, helps
the listener to focus, renews his interest and avoids the boredom that may
result from always adhering to one style. 

Haazim
said in Minhaaj al-Bulagha’: They become bored when the style
persists in using the first person or the second person, so they move from
second person to third person. By the same token, a speaker may change the
pronoun and play with the words, sometimes using the first person to speak
of himself, and sometimes using the second person or third person. Hence
speech that persists in using a particular pronoun, whether it is the first
or second person, is not regarded as good; rather it is more appropriate to
move from one to the other. 

Then
az-Zarkashi (may Allah have mercy on him) gives different examples of such
usage and their effectiveness in conveying the message. 

See:
al-Burhaan fi ‘Uloom al-Qur’an by Badr ad-Deen az-Zarkashi, 3/314-330 

For more
information, please see the answer to question no. 606 

And Allah
knows best.