Given that they're migratory, I'd buy the zooplankton explanation, however, I'm also curious if it doesn't have something to do with courtship.
There is at least one satellite tagging program, so hopefully that will give us more data along side the citizen science (cool!) project.
Sort of tangential, but too cool to not point out. Basking shark corpses are one of the main suspects in sea monsters that wash ashore.
I have been thinking that it might have to do with a breeding ritual myself, and it's cool to see that multiple people have also been thinking about this. I tried to find out what I could about their breeding habits, and it does not seem like we really know anything about it, so it is absolutely plausible.
It's great to see that there's a satellite tagging problem in place. I bet we will learn a lot more about these sharks in the next decade with all this research going on.
That's a cool fact. And it makes a lot of sense that these could be mistaken for "sea monsters" after having decomposed to a certain extent.