Same I'm also do not understand, Elche is the team which is at the bottom of the standings right now, and yes it should be easy for Atletico Madrid. But somehow they can lose, even statistically Elche in that match is better rather than Atletico Madrid.
Atletico really underestimate Elche.
Moreover Atletico also didn't play as best as they are usually. Tjey cound't domimate the match and not really ready with the match. Seems like to be this. That is why theybwere lost.
Btw, because if Atletico loss and Real Madrid victory, now Real Madrid becomes on their second position as runner up in La Liga.