Photographer published a photo on their own website.
Tweeter posted it on Twitter. Twitter trimmed it, removing the credit line.
Retweeter retweeted it. Photo was still trimmed.
Photographer sued Retweeter for distributing their work without proper credit.
An obvious question would be if Photographer asked them to delete the tweet(s) first, and why they chose to sue Retweeter not Tweeter. I'm guessing it's because the photographer is seeking financial compensation and the retweeter looked someone financially capable. This disclosure is a preliminary step to further lawsuits demanding remedies.