wrong question. should be where did disrespect of this country begin? Not respecting the Anthem is just symptomatic of a far bigger problem.
Even Southerners, who fought a war to achieve independence and thousands dying to do so, still overall are willing to respect this country and its flag.
So the search goes deeper.
I believe it is a number of things, which can include
the failure of this country's leadership to maintain a solid bedrock of morality which honors the time-tested family, particularly that of a father integrated within the family unit.
the turning away of the morality of a God-fearing nation which recognizes that our rights are those bestowed by God, not some government.
the absence of meaningful civics classes taught in our school which remind new citizens how and why our country was founded, which is different from any other on earth since history began. That absence has caused otherwise normal people to reduce out heroic Founders to vermin instead of heroes.