Yeah, it was more that the team struggled a bit, and Klopp seemed to have lost his way, which was exactly what happened at Mainz; with the 05ers, they were very much punching well above their weight when he was there, and he felt he couldn‘t keep that up, so he walked.Didn't he have a really good Dortmund side then they just collapsed, never recovered and almost got relegated - so he resigned. That would be ideal to happen at Liverpool.
edit: to answer my own question no that didn't happen, but they were in the relegation zone in February
At Dortmund, he‘d done a decent job, but they were in a need of a rebuild; Klopp wanted to spend-spend-spend, and the club, mindful of nearly going bust a few years earlier, instead wanted to build around younger players who could then be sold on after 2-3 seasons - a pattern they have continued recently with considerable success, with the likes of Jadon Sancho and Erling Haaland going for huge sums, and Jude Bellingham (I can pretty much guarantee you) will be the next high-profile younger player to leave for big money).
Klopp wasn‘t interested in a lengthy project like that, so again he quit.