Because Moses had asked to see God, God revealed Himself to Moses.
Following the golden calf incident, Moses asked to see GOD.
Though GOD could not fully grant Moses' request (because Moses would die from seeing His face), GOD told Moses He would let Moses see His back.
1 Now the Lord said to Moses, “Cut two tablets of stone like the first ones, and I will write on these tablets the words that were on the first tablets you broke. 2 So be ready in the morning to come up on Mount Sinai and to present yourself to Me there on top of the mountain. 3 But no man shall come up with you, and let no man be seen throughout all the mountain; let neither sheep nor oxen feed before that mountain.” 4 So he cut two tablets of stone like the first ones. Then Moses rose early in the morning and went up to Mount Sinai as the Lord commanded him; and he took in his hand the two tablets of stone. 5 Now the Lord descended in the cloud; and He stood with him there, and he called upon the name of the Lord. 6 Then the Lord passed before his face and proclaimed, “The Lord God, compassionate, merciful, longsuffering, abounding in mercy and true, 7 preserving righteousness and showing mercy unto thousands, taking away lawlessness, wrongdoing, and sins; and He will not clear the guilty, visiting the lawlessness of the fathers upon the children and the children's children to the third and the fourth generation.” 8 So Moses made haste and bowed his head toward the ground and worshiped. 9 Then he said, “If now I have found grace in Your sight, let my Lord go with us, for the people are stiff-necked; and You will take away our sins and lawlessness, and we will be Yours.”
29 Now when Moses came down from Mount Sinai (and the two tablets were in Moses’ hand when he came down from the mountain), he did not know the skin of his face was glorified while God talked with Him. 30 So when Aaron and all the children of Israel saw Moses and the glorified appearance of the skin of his face, they were afraid to come near him. 31 Then Moses called to them, and Aaron and all the rulers of the congregation returned to him; and Moses talked with them. 32 Afterward all the children of Israel came near, and he gave them as commandments all that the Lord commanded him on Mount Sinai. 33 When Moses finished speaking with them, he put a veil on his face. 34 But when Moses went in before the Lord to speak with Him, he would take the veil off until he came out; and he would come out and speak to the children of Israel whatever the Lord commanded him. 35 So the children of Israel saw the face of Moses, that it was glorified; and Moses put the veil on his face again until he went in to speak with Him.
(Exodus 34:1-9, 29-35 OSB)
In this situation, GOD's glory radiated outward and onto Moses as GOD's attributes and character were proclaimed.
This event seemingly produced a natural reaction in Moses to fall down prostrate, which, by definition, is worship.
Why is this?
Because when a human perceives the vastness of GOD's greatness, our natural reaction is to compare GOD with ourselves. In that instant, we remember how small and insignificant and unworthy we are. The natural reaction to that realization is to fall down, to make oneself low, to humble ourselves.
In Moses' case, the closeness with which he came in proximity to GOD's greatness produced a glow upon Moses' face.
As Moses returned to the people, the glow persisted, temporarily. And it made the people afraid. Therefore, Moses put a veil over his face when he spoke with the people.