TLDR; Basically an attacker owns a bunch of TOR nodes, and then he DDoS' the nodes he doesn't own until the hidden service is forced to utilize the attackers compromised nodes.
I recently posted in the TOR subreddit asking how DDoS could be used to deanonymize hidden services. If anyone is interested in this topic here's the link to my thread there (EDIT: the people who replied there have much more technical knowledge than I do and really explain this stuff well):
https://www.reddit.com/r/TOR/comments/7lt954/ddos_related_deanonymization_techniques/
I fully believe that the DDoS attacks we are seeing on markets and vendor shops are being carried out by certain government agencies in order to deanonymize them.
Here is a link to documentation on how a similar attack to this could be carried out by an attacker with limited resources:
https://www.ohmygodel.com/publications/sniper-ndss14.pdf
In conclusion I believe that this is just one more thing that will push the markets towards the decentralized options. I also believe that established vendors could in the future move away from having a site/shop of any sort at all. Perhaps a better way for them would be to have a "mailing list" in which they email an inventory/pricelist to those who have signed up and then do all DD's through an email service accessed through TOR.
Wasn't there speculation that this method was used for operation onymous?