D.E. Aune has three explanations for the proliferation of N.T. pseudonymous writings:

  1. it arose at a time whent he biblical canon was already closed and well-known names were used to secure acceptance
  2. it was used to protect the identity of the writer who might be in danger if his or her true identity were known
  3. apocalyptic visionaries may have had visions from the figures to whom they attributed their work

Aune also lists seven categories of pseudepigraphy (N.B. Aune questions the authorship of some canonical works):

  1. works not by an author but probably containing some of his own thoughts (Ephesians and Colossians)
  2. documents by someone who was influence by another person to whom the work is ascribed (1 Peter and maybe James)
  3. compositions influenced by earlier works of an author to whom they are assigned (1 Timothy, 2 Timothy, Titus)
  4. Gospels (eventually) attributed to an apostle but deriving from later circles or schools of learned individuals (Matthew and John)
  5. Christians writings attributed by their authors to an Old Testament personality (Testament of Adam, Odes of Solomon, Apocalypse of Elijah, Ascension of Isaiah)
  6. once-anonymous works now correctly (perhaps Mark, Luke, and Acts) or incorrectly credited to someone (some manuscripts attribute Hebrews to Paul)
  7. compositions that intentionally try to deceive the reader into thinking that the author is someone famous (2 Peter)

Source: D.E. Aune, Prophecy in Early Christianity, cited by L.M. McDonald in The Biblical Canon.