Krishna initiating contact when he's in front and running towards the ball?
The defender runs behind and across Krishna and his leg inadvertently knocks Roy's foot, which causes him to trip. That's interfering with an opponent. It happens many many times in a season and is always penalised. This is only different because it's ended in a penalty rather than a free kick.
As I see it, Alessi running behind Krishna and causing him to trip is 'acting without precaution' - which doesn't require Alessi to be making a challenge to be penalised.