If you pull your head out of a challenge with someones boot, it's a foul without contact. If you just lightly clip a players feet when he's running, he'll still likely go down under minimal contact.
Niasse is trying to run through the middle of 2 Palace players, and Dann has obstructed him after the ball has gone past him. When you've been hit and you are off balance, a players natural reaction is not to carry on running until they hit the floor, everybody braces or prepares for the fall.
Smaller players are more likely to get these decisions, but it doesn't take a lot to unbalance. If it's a player on the floor, then you'd be expected to avoid as player on the floor doesn't necessarily now much about it. But if a player leaves a leg stuck out across a players path or barges you as you get there, he's trying to foul and gain an advantage from contact.
Niasse is trying to squeeze through a closing gap, and if it was only Dann, he'd have gone around him. But he didn't initiate contact, Dann knew what he was doing by trying to block him off. It's soft and maybe not a pen, although I think it's a foul pretty much anywhere else on the pitchor if a striker blocked off a defender, so team mate got clear run at goal.