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.
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 :/
There was also a big displacement of black voters in LA between 2004 and 2008 resulting from Katrina. But yeah that’s probably part of it too; it’s even more dramatic if you look at Arkansas, which Kerry came within single digits of winning and where Hillary was polling well ahead of McCain, yet Obama got blown out.
I don't think there were too many of these voters.
My entire county was a Bush/Bush/Obama county and I personally know many such voters including multiple family members. Mostly educated white suburbanites. The shift of that demographic leftward was accelerated significantly by Trump but it was already well underway by this point.
Plus how exactly do you think Obama won the presidency after two Bush wins, including even a state like Indiana where Bush beat Kerry in a landslide, without many voters switching from Bush to Obama?
There definitely were some, but no where near the 9 million or so Obama 12 Trump 16 voters. But there were a number of Bush-Bush-Obama voters in the Midwest.
Obama won in 2008 due to exhaustion with the GOP after Bush II, Working Class voters still having a lot of loyalty to the Democratic party, and Obama bringing out and engaging a lot of new voters who had become politically aware during the Bush II Presidency. He won Indiana thanks to an unprecedented ground game and voter registration drive. The GOP stood little chance in 2008, the Recession really broke open the floodgates for the Democrats to gain a lot of ground.
Too bad they couldn't sustain it. 
2004 -2008 was a R+2 -> D+7 shift
2012-> 2016 was a D+4 -> D+2 shift
You can't seriously tell me the population grew so much over Obama that such a minor shift could actually mean more voters.
