#3 Fatigue
Honestly, this is probably the most accurate reason for Real Madrid and Barcelona's troubles this years. It's a perennial cycle in football - top clubs experience a dip in form after a summer with major international tournaments.
This summer was unique in that both the Euros and the Copa America took place, which basically guaranteed that every starting member and top 5 substitutes of Barcelona and Madrid spent the summer going all out for their nations. With so many players not having the best of pre-seasons, it's no surprise that Madrid and Barcelona have dropped so many points already.
Another tactical issue is that the way Real Madrid and Barcelona setup to play, any mental fatigue inhibits the consistency of their attacking play. It was visible in Barcelona hammering Celtic and Leganes one day and losing to Alaves another day. In contrast, Atletico Madrid are very used to playing attritional football, and have absorbed a gruelling summer to top the La Liga table