They buy up expired domains and immediately put them up for sale for $XXXX
At that price, they can sit on the domains for decades and still make a profit when it's sold.
Eg: https://www.hugedomains.com/domain_search.cfm?domain_name=cs...
In the end I went with an alternative TLD. it isn't as good, and I'd have been willing to pay a decent amount for the one I wanted (even their original price, reluctantly), but I refused on principle when they got silly.
They are just parasites.
I feel like I can never let a domain that I may want to use again in the future ever laps or someone will buy it and extort some exorbitant price from me to get it back.