https://www.pcgamer.com/hearthstone-got-its-fatigue-mechanic-thanks-to-jeff-kaplan/

Hearthstone director Ben Brode shed some light on a few Hearthstone mysteries in a new video from Ars Technica. The whole video is a good watch if you're interested in Hearthstone's history and DNA, but the best bit is right at the end, where Brode explains how Hearthstone got its fatigue mechanic, which deals escalating damage to players each time they draw cards after their deck is empty. As Brode explains, fatigue damage didn't always exist, and they only came up with it after Overwatch director Jeff Kaplan played Hearthstone for himself. 

"We were doing a playtest and we asked Jeff Kaplan to come over and try the game," Brode said. "And he got really into it, he started playing minions and attacking, he was having fun figuring out what cards to play. He had never played it before, he was excited to get into it. And he was very close to winning, and then the computer he was playing against passed the turn. And it said 'Your turn, you lose!' And he was like, what?"