They think it's loss of Arctic sea ice:

Unlike the two previous events, a historic loss of Arctic sea ice could be to blame for the latest gray whale die-off. That's because sea ice hosts a carpet of algae on its underside, which decays and showers the seabed with food for bottom-dwellers, including the whales' preferred crustaceans.

"With less ice, you get less algae, which is worse for the gray whale prey," Stewart said. Melting sea ice also frees up passage for strong currents that sweep away the sediment and leaves bottom-dwelling crustaceans and other creatures homeless. "All of these factors are converging to reduce the quality and availability of the food [gray whales] rely on," he said.

Climate change might be the reason this mortality event is dragging on for longer than the previous two, Stewart said. "What we're seeing is much more of a bumpy ride in response to highly variable and rapidly changing ocean conditions," he said.

Didn't see that OP had already written a comment with the explanation, please excuse me.