It wasn't at any one point. They bled them fairly consistently from Katrina, Iraq dragging on, the financial collapse and probably lost a few as early as Bush's botched Social Security privatization attempt.
Bizzarely, Obama performed worse than John Kerry in Louisiana in 2008. Kerry won 42.3% while Obama won 39.9% and barely didn't break 40%.
My only guess is that there was some racial resentment from rural whites. Despite Hillary endorsing his campaign, it seemed that a decent chunk of her southern white supporters (many of them voted for her husband Bill in 92/96) stayed home or even switched to McCain in the general :/