Is the asphalt itself crumbling or is the sealcoat turning into 'particulate'? There is no way sealcoating would cause the asphalt itself to deteriorate but we have had sealcoating turn into 'black sand'. The company that did the job redid it at their expense. The reason given was that the weather following the coating was 'bad' (too cold, too wet, too windy) and did not allow the sealcoat to cure and bind with the asphalt. As Imogene posted, call the company, have them come out and verify if the sealcoat did not cure and is coming up and if they are ethical, they will redo the job.