What is the actual thinking coaching wise behind scoring early and reacting by having a low block for the rest of the game?

Is it an actual tactical decision the manager has laid out as a possible scenario or a mentality problem of the players?

If the former, why, does it work most of the time and we only hear about it and talk about it when it doesn't?

It's not even like Mourinho where you could knick a goal or two but remain consistent and it pays off, I'm thinking in particular about Sunderland in the playoffs and the Euros final.

I'm not saying everyone should play like us and Leeds but it baffles me sometimes but I have no coaching experience.