The many factors that have resulted in the position we find ourselves in have been discussed extensively on this forum. These include:
World Cup hangover
Injuries to key players (could be linked to the World Cup)
Physical Fatigue
Bad luck (or opposition good luck)
Lack of on-field leadership (how many of our players have captained or vice captained at either club or national level? Liverpool and Spurs, on the other hand have Milner, Henderson, Van Djik, Robertson, Keita, Mane, Salah, Lloris, Kane, Erikson, Son who've all captained or vice captained somewhere in the last 18 months. This is key, especially when things aren't going your way in a game)
Mental fatigue
The last factor could be linked to the exertions of the World Cup and indirectly linked to how Pep approaches the game. He's a perfectionist who places specific demands on his players. He's after footballing perfection. His methods don't allow for any coasting and require players to be mentally switched on 100% of the time. The results of that approach were evident last season as we swept sides and broke all sorts of records along the way. However, it's not humanly possible to be switched on like that forever. The players are not machines and over time this can wear them down. Factor the World Cup in and you'll see that over the past 18 months, the players have had no real mental break from the game and its exertions. And I personally think that's one of the biggest reasons why many of our players seem to have lost form "all of a sudden".