Similar to the practice of law, the standards and practices of accountants will change based on an ongoing interpretation and application of tax law and codes.
Accounting interpretations are generally publications from groups like the Financial Accounting Standards Board (FASB)and the American Institute of CPAs (AICPA). These interpretations are not official standards, and do not have to be followed the way a standard does, but they give insight and suggestions for situations which may be new developments.
Accountants have to deal with businesses and individuals from all industries and walks of life, and it is inevitable that the laws and finances pertaining to this melting pot of clients will change as time passes.
Accountants who used to do things one way will probably need to start doing things another way at some point, and this is part of the reason that law requires CPAs to complete continuing education classes throughout their careers. Official standards will sometimes be updated based on interpretations.