“For today the L-RD will appear to you.” – Leviticus 9:4
Upon completion of the (tabernacle), offerings were brought for its inauguration: these included an offering to atone for Aaron’s role in the golden calf incident. Commentary reads, that the offerings were prepared; however, the fire had not yet descended from the sky; so, Aaron grew concerned. His guilt in the sin of the golden calf compelled him to think that the delay was a sign that he was not completely forgiven.
At this point, Moses and Aaron entered the sanctuary. While no reason is given on the actual passage found in Torah, commentary offers several reasons. One reason given is that Aaron confided in Moses, concerning his felt shame over his sin. Thus the two of them entered the sanctuary, in order to pray to the L-RD to forgive Aaron. When they walked out of the sanctuary after praying, the fire descended upon the mizbeach, consuming the offerings.