Radak, Malbim: Their backs were towards Heichal Hashem, and their faces faced forwards - the opposite of Hashem's intent. Hashem commanded to put Pesach ha'Mishkan in the east and Kodesh ha'Kodoshim in the west, to nullify sun-worship that people were used to at the time. They will enter the Mishkan [in the morning] with their backs to the sun, to erase that Emunah from their hearts. And so He commanded in the Beis ha'Mikdash. They were Mevatel the Mitzvah and the intent. They turned their faces away from Avodas ha'Elokim, and from bowing to His Beis ha'Mikdash, and returned to serve the sun and bow to it.