No one really knows. It seems to be a combination of global warming, ethical fishing, and declining cod stocks.
There may be another factor at play, though. Heavy fishing not only shrank cod numbers; it also shrank their average body sizes. Research suggests the average cod size fell 20% between 1997 and 2007. A recent study by Rick Wahle and colleagues linked the lobster boom more closely to the shrinking sizes of predator fish, than the population decline itself.