Refferning domains boost and decline coincides with the March 2019 Core Update

[Hacedor]

Hi.

An escort site asked me to try and help them understand what went wrong and they disappeared from SE results.

For one thing they haven't done any SEO so that's that.

Appart from that, they started declining from the Google March 2019 Core Update and on.

Studying their case, I see that at the time of the Core Update and the start of their decline, there was a boost of refferning domains. Check the graph:



Why is that, do you think?



For the rest most refferning domains are trashy ones from India ( .in ) with alexa ranking listings. Here are some:
http://www.janhit.in/
http://www.officialwebsites.in/
http://www.onlinenewspaper.in/



What's more they all seem to be all copies of the same. Here's an example:


Any idea whether this can actually cause damage?

I'd appreciate any feedback.

Thank