Hello, I don’t know if it occurs always, but last time someone played leeroy jenkins against me, the Red arrow that shows the target of the attack departed from my opponent’s hand and not the minion itself. I am sure about it because the opponent was nice enough to stall the game until the end of the turn for that single decisive final blow, almost 75 seconds of that long arrows on my screen 
Sounds like a temporary disconnect from your side.
Your opponent played quickly. He played Leeroy and then attacked.
Since you have been partly and temporary disconnected, the red arrow shows the players hand where Leeroy was before instead of the board - where Leeroy already was when he attacked.
Suggest to read below to understand how connections work in HS:
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