It is based on the understanding that some aspects of the Prophecy given by Ezekiel against the King of Tyre are more correctly directed at the demonic influence empowering that King identified to be Satan himself and the influence the KJV has had on the formation of Pentecostal doctrine. Consider:
12 “Son of man, raise a lamentation over the king of Tyre, and say to him, Thus says the Lord God:
“You were the signet of perfection,
full of wisdom and perfect in beauty.
13 You were in Eden, the garden of God;
every precious stone was your covering,
sardius, topaz, and diamond,
beryl, onyx, and jasper,
sapphire, emerald, and carbuncle;
and crafted in gold were your settings
and your engravings.
On the day that you were created
they were prepared.
14 You were an anointed guardian cherub.
I placed you; you were on the holy mountain of God;
in the midst of the stones of fire you walked.
15 You were blameless in your ways
from the day you were created,
till unrighteousness was found in you.
16 In the abundance of your trade
you were filled with violence in your midst, and you sinned;
so I cast you as a profane thing from the mountain of God,
and I destroyed you, O guardian cherub,
from the midst of the stones of fire.
17 Your heart was proud because of your beauty;
you corrupted your wisdom for the sake of your splendor.
I cast you to the ground;
I exposed you before kings,
to feast their eyes on you.
18 By the multitude of your iniquities,
in the unrighteousness of your trade
you profaned your sanctuaries;
so I brought fire out from your midst;
it consumed you,
and I turned you to ashes on the earth
in the sight of all who saw you.
19 All who know you among the peoples
are appalled at you;
you have come to a dreadful end
and shall be no more forever.” - Ezekiel 28:12-19 ESV
It doesn't appear to be talking about just a man (despite that exact sense being given to the preceding verses). The particular sense of worship leader has come from translation of some of the obscure words in verse 13:
13 Thou hast been in Eden the garden of God; every precious stone was thy covering, the sardius, topaz, and the diamond, the beryl, the onyx, and the jasper, the sapphire, the emerald, and the carbuncle, and gold: the workmanship of thy tabrets and of thy pipes was prepared in thee in the day that thou wast created. - Ezekiel 28:13 KJV (emphasis added - cf. other versions)
This has been taken to mean that musical instruments have been incorporated into the very form of the 'guardian cherub' which seemed to have a special place on 'the holy mountain of God' hence the inference that the being referred to was the leader of worship in heaven. That it's identity is that of Lucifer is seen more from comparing this passage and in particular verse 17 with Isaiah 14:12-14 (cf. also this on-line article).