I've had it happen to me twice now since the patch that I've had a creature go to the graveyard for no apparent reason.

The first time I was playing a blue/red prowess deck and had my opponent attack into my Abbot of Keral Keep and my summoning sick Pyrehound, then before blockers (I know :/) I played expedite on my Pyrehound. All the Prowess triggers resolved and when finally the Expedite resolved the Pyrehound went to the graveyard with the lightning zap sound from Expedite. Needless to say, I was baffled. I'm about 75% sure it was a ranked match, not an AI match.

The second time happened just now vs AI while I was grinding my two dailies ( 1/ 1 counters and produce tokens) with my W/u Humans deck. I had clear control of the game with 2 dozen creatures while the AI had none. He cast a Friendly Stranger and as any turn I attacked with my Westvale Cult Leader and he blocked and it died. The next turn, he cast another Friendly Stranger, but instead of entering the battlefield, it just went straight to the graveyard. What.

Edit: Thinking about it, the second one might've been caused by a Jace, Unraveler of Secrets emblem.

TL;DR: Pyrehound died from Expedite resolving, Friendly Stranger went to the graveyard instead of the battlefield after being cast.