Why does the worshipper prostrate twice in each rak‘ah?

What you say is not correct and it is not permissible to mention it or speak of it for a number of reasons: 

Firstly:

It is a claim for which there is no evidence. The books of tafseer are readily available and none of their authors said such a thing. 

Secondly:

In His Book Allah, may He be exalted, only mentioned one command to prostrate to Adam, then He said that the angels all prostrated except Iblees, who was one of the jinn; he rebelled against the command of his Lord and refused and was arrogant, and thus the test was completed. 

Allah says (interpretation of the meaning):

“And (remember) when We said to the angels: “Prostrate yourselves before Adam.”. Snd they prostrated except Iblîs (Satan), he refused and was proud and was one of the disbelievers (disobedient to Allâh)”

[al-Baqarah 2:34]

“And (remember) when We said to the angels;”Prostrate yourselves unto Adam.” So they prostrated themselves except Iblîs (Satan). He was one of the jinn; he disobeyed the Command of his Lord”

[al-Kahf 18:50].

Thirdly: 

The prostration of the angels was to Adam (peace be upon him): “‘Prostrate before Adam’”. As for our prostration in prayer, it is to Allah, may He be exalted. There is no connection between the prostration of the worshipper in his prayer and the prostration of the angels before Adam (peace be upon him). 

Fourthly: 

There is nothing in the Qur’aan or in the Sunnah to suggest that when Iblees refused to prostrate to Adam, Allah got very angry in a way that alarmed the angels. So it is not permissible to attribute such anger to Him in this situation. It is not permissible to claim such a thing except with sound evidence. 

It should be noted that Allah, may He be exalted, has forbidden us to speak of Him and His religion without knowledge. He says (interpretation of the meaning):

“He [Shaitân (Satan)] commands you only what is evil and Fahshâ (sinful), and that you should say against Allâh what you know not”

[al-Baqarah 2:169]

“Say (O Muhammad ): “(But) the things that my Lord has indeed forbidden are Al-Fawâhish (great evil sins, every kind of unlawful sexual intercourse,) whether committed openly or secretly, sins (of all kinds), unrighteous oppression, joining partners (in worship) with Allâh for which He has given no authority, and saying things about Allâh of which you have no knowledge”

[al-A‘raaf 7:33]. 

Al-Daarimi (174) narrated from Abu Moosa (may Allah be pleased with him) that he said in his khutbah: Whoever has some knowledge let him teach it to the people, but let him beware of saying that of which he has no knowledge, lest he transgress the limits and go beyond the pale of Islam. 

And Allah knows best.