It is entirely possible it is in some way related to human activity. It is also possible that it is completely unrelated. Booms, busts, and radical ecosystem changes do happen all the time in nature.
Nature isn't gradual, it's catastropic. The Younger Drayas caused a 7° to 10° C drop worldwide in a matter of decades.
Volcanic eruptions, sudden drought, earthquakes, asteroid impacts, abrupt population crashes: sudden change is normal.
Yes, a lot of the change we see now is human impact. But this could easily be a virus that hit the sea stars out of the blue. We just don't know.
If you've been watching the changes in the aquatic habitat and the sea life populations they becoming simple / basic and more homogeneous as a function of changing water temperatures (and harvesting of sea life without the ability to replenish).
I won't disagree that it is possible virus or range of other possibility - I am not providing an answer. However it's a very high probability that the natural ability for that ecosystem to defend against the virus (if it was) was likely impeded by human impacts which change the water temperature and the species of life that live in that area.
We could posit many different theories, the fact is that nature is changing rapidly under our watch and to insinuate that it is a natural occurrence is disengeinuine and irresponsible at best and seems to be the push of a false narrative to undermine the consensus in the scientific community.
That being said, it's almost definitely due to humans, I just wanted to be pedantic.
It would be very surprising, to me at least, to find ultimately humans did not contribute to this chain of events. Whether it's something like microplastics causing the lesions on the star fish or water temperature/chemical concentration leading to some fungal spore growing, I suspect ancient kelp forests aren't vanishing without our help.