Failure to handle the dressing room issues made him lose focus on taking the club forward and he further lost his job, so if a similar thing repeat in Leverkusen, let's say some players become toxic and disagree to his tactics maybe a similar thing could occur and caused him his job too in Bayer Leverkusen.
Don't say I'm dwelling in the past dear, both past and future operate together you can't talk about Ten Hag's record without including his good and bad ones, I'm not wishing him bad, I might be wrong that he won't win this season's trophy with Leverkusen but I strongly believe that Bayern Munich is more capable of winning it this season.
Well it was simple, whenever they lost, ETH made it known to the public it was the players fault that they lost. If you do that, of course you are going to lose the locker room, because nobody on the locker room will respect you. Not the players, not the other staff, and certainly not the fans.
A real manager should know that you are the leader, and even if it's not your own fault, you go out there and tell the media that it's your own fault and you will try to do better, that's what a leader does. Doesn't matter if it's really your fault or not, a leader takes the blame for being bad, and he didn't. This is why it's really not a smart move to throw the players under the bus whenever things go wrong, and it's bad look to say "I won cups, I am good, you are making a scene for one loss" when you are doing badly, just say "we are sad, we will try harder, and will be better", simple media training.