He left his position in the centre of the pitch to attempt to cut out a pass from the opposition right-back to the centre back 30 yards away, with two Barca players already close by. Thus creating both a passing lane and acres of space for the opposition to exploit that wouldn't have been there if he didn't mindlessly charge up the pitch at that moment.
Especially with minutes left and only a small lead.
I explained this situation here before.
It blew up because someone posted it on reddit in isolation, and people look for easy explanations and simple memes.
The clip starts just after Coutinho had misplaced a very easy pass, and the Espanyol player is immediately under pressure by three players, with the only way out being a back pass to his RB. Puig immediately spots this, and sees that Depay is hesitating to cover that pass so he takes a quick decision in order to win the ball or at the very least force the RB to hoof it.
You can't defend with the assumption that the one pressing will be nutmegged!
You can say that with 1-0 and a few minutes left, you should play extremely safely and not leave any gaps etc, but I think it goes against the mentality needed - they should play their way all the time and not allow a team like Espanyol to push them down, but try and win the ball!
de Jong was playing pivote then, but stood and marked someone in the back line, Pique stood lazily very far to the right, and Garcia helped with the pressing. With Puyol and Busquets there, the back line would rise and Busquets would pressure the player that recieved the nutmeg immediately. de Jong has this tendency to fall and fall, instead of taking a step forward.
How much talk there has been about taking a step forward after losing possession, how it has been key for Barcelonas entire way of playing!
Check for example Fabregas in this clip at 0:40, or even Thiagos immediate intention later in the clip:
https://www.youtube.com/watch?v=fc_ljII-Vuc
This is btw one situation from the game where Puig arrived to play 20 minutes and didn't lose the ball once, and won at least one loose ball that would otherwise lead to a counter, because of his good positioning and awareness.
Frenkie lost the ball and caused the free kick that left the Espanyol player with a completely free header because Frenkie didn't mark him... Just as an example...
Xavis mission is to get the players to play together and move as one unit. I hope that Puigs aggresiveness and willingness to press and win the ball will become a guide for the rest of the team. Gavi has some of it too, and a lot of the other players too, but they have been more and more accustomed to fall back instead, because they haven't trusted each other.