The latest reports out of California have it that the Brown Pelicans turning up dead or dying along the coast of that state may have succumbed to bad weather.

Brown Pelican, much like those affected by the die-off in California
The birds had been spending time off the coast of Oregon when a severe storm hit, driving the birds south and inland, fleeing the winds and freezing temperatures. Many of the birds recovered by rescuers showed signs of frostbite on their feet and pouches, suggesting that the birds were indeed unprepared for the unusually harsh conditions. As many of the birds were in good body condition, it does not appear that the birds had been suffering from a protracted illness and may simply have been caught in the wrong place at the wrong time.
The diagnosis of this event has been a good example of multiple agencies and wildlife groups working together to determine the cause of an unusual mortality event, and also shows that sometimes these things stem from the most quotidian roots.
Thanks so much for the follow-up and update on this event !