Why? Lots of their supporters actually think they're Real Madrid. You know with how they have a top 5 b2b midfielder, only team to challenge Chelsea for the title right now and the fact that they don't need any signings right now because their kids are La Masia part 2.
And lots of their supporters think doom is coming and they'll finish 6th.
It doesn't make it true.
I'm not interested in what "lots of supporters" think, football fans in general know very little about football, especially ones who never played or were never coached at any level.
Fatigue and playstyle has most to do with Liverpool's problems in a season that's nevertheless very, very good.
There's a reason why they've won most of their matches vs the top teams and not lost a single one; and why they can struggle against mid-table and bottom teams if things don't go their way early.
The high pressure leaves you open at the back - and while that's much less the case vs team that want to attack as well (like ManCity or Arsenal), there's always a higher chance you'll leave your defenders exposed with no support vs teams like West Ham or Leicester (if the result isn't in your favor).
Having Mane and Matip back will help, both offensively and defensively.
Matip is a calming influence at the back and will allow Lovren to go back to his proper side (he's playing his "off" side with Klavan).