Last night I had a strange occurrence in an Arena Powered Cube game.

During the end step of my opponent's 2nd turn, I resolved a Mystical Tutor and put Time Walk on top of my deck. When my 3rd turn started, the Time Walk went to the graveyard and I drew a new card, despite neither player having done anything. I've outlined the steps of what occurred below in Reproduction Steps.

My T1: Land, pass.
Opp T1: Land, pass.
My T2: Land, pass. (holding up Mystical Tutor).
Opp T2: Land, pass. During end step, I cast Mystical Tutor, and put Time Walk on top.
My T3: Time Walk gets milled???? And I draw a new card (Steel Seraph). Neither player cast anything or had any kind of effect that would've milled a card.