The incident happened during the 13th over when Ishan Kishan flicked a Rashid Khan’s delivery towards the keeper's end. In process of collecting the throw from Vijay Shankar, Bairstow accidentally bumped into the stumps and dislodged the bails before grabbing the ball.
After collecting it, he disturbed the stumps once again, in such a way that the umpires found it legal to give Ishan Kishan out.
Here’s the video:
Why the 3rd umpire gave Ishan out?
According to Law 29.1, if the bails have come off the wicket, the fielder still has the option of uprooting a stump, ‘providing that the ball is held in the hand or hands so used, or in the hand of the arm so used.’
After collecting the ball and dismantling the stumps, Bairstow made sure that he should apply enough force that stumps should come entirely out of the ground.
At that point of time, Kishan was a few inches short of making his crease, leaving the third umpire a fairly straightforward decision to make.