Why did Nuri Sahin struggle at Borussia Dortmund?

Bundesliga club Borussia Dortmund have sacked coach Nuri Sahin. The club is now looking for their eighth coach in the last ten years.

source https://www.dw.com/en/why-did-nuri-sahin-struggle-at-borussia-dortmund/a-71368743?maca=en-rss-en-ger-1023-rdf