Most likely the root cause is acidification of the ocean. Increasing the carbon dioxide in the ocean decreases the ph level of water, making it more acidic. While some animals and plants have evolved to handle highly acidic environments, most have not, and in general acid tends to be bad for living creatures. I know I'm oversimplifying things, but this seems reasonable. I assume if there were giant balls of plastic pollution in the whales belly this would be found in an autopsy and therefore plastic pollution is probably not the root cause.