Since we're all speculating, I would guess that ocean acidification could be at play here. The arctic regions are carbon sinks so they experience acidification earlier than other parts of the ocean. When the CO2 is absorbed into the ocean it causes the pH level to decrease, making the waters more acidic. This inhibits CaCO3 (calcium carbonate) formation found in many ocean species like crabs.
If there's a rapid die off of this magnitude there aren't many other problems which would match this level of impact. It could be something else environmental like the water temperature but it doesn't seem like an anthropogenic problem. Similar problems have happened already for other species like oysters.[0]
I was an intern with NOAA in a fisheries lab studying ocean acidification back in college in 2011. There was a large die off in oyster farms the year before which wiped out a good size of their production. The issue then was that the farms were cycling the water in directly from the ocean into the tanks without adjusting the pH. As the pH in the ocean water dropped, it killed off all of the oysters in the farms all at once. They had to do a quick about face to treat the water to regain the catch.
Ocean acidification seemed like a problem that would get worse and worse over time. The estimate back then was that it would take roughly 50 years for the CO2 in the air to equilibrate with the CO2 in the ocean. Since we won't stop emitting any time soon it's going to get a lot more acidic.
I'm a decade behind on the status but back then it seemed like there were a lot of unknowns around the problem too. I'm not sure if more has come to light on the impacts it might have for ecology or the biochemistry of different species. It was a complex problem so predicting the outcome seemed extremely difficult. Hopefully more is known but if not surprises like this would likely crop up. For shellfish, once it goes under a certain pH threshold for a species the young will be unable to form shells which can cause them to die en mass in a short time period.
It doesn't seem like a problem that we can engineer our way out of. The best we can do is probably to monitor and perform science experiments to see some of it coming. Then try to mitigate the damage.
It seems like NOAA would have been able to call out the calcification problem for the snow crab in advance though since there have been issues with other crab species.[1] They probably would have detected the pH change in the waters too so this might not be the cause. Either way it's an interesting subject to learn about.
[0] https://blogs.oregonstate.edu/breakingwaves/2011/06/28/ocean...
[1] https://research.noaa.gov/article/ArtMID/587/ArticleID/2581/...
NOAA's ocean acidification program
https://oceanacidification.noaa.gov/Home.aspx
This was a good 2011 report from the IPCC if you're interested.
https://www.ipcc.ch/publication/ipcc-workshop-on-ocean-acidi...