Remember that there are many voters in the Deep South and in Appalachia who are officially registered to the Democratic party, but generally vote for Republican candidates for most federal offices. These people don't like how the Democrats have become liberal on most social issues.
Texas doesn't have party registration. It's open primary.
You could be on to something, as there was a primary for the Senate race on the same day as the Obama/Clinton race. I still find that unlikely because the 21 votes Obama got were still more than Bush and Gore's vote totals. Put simply, they would have been self-identified Republicans by then.
Also, Kerry got exactly 18 votes in the 2004 Democrat primaries and exactly 18 votes in the general election. That's strange too, but the drop in votes Obama received are more interesting because Kerry didn't start fading until the RNC convention.