a) in Shakespeare’s case perhaps the most important was „fear of persecution“: Any author in the Elisabethan era who committed secret betrayal or confessed confidential state or religious interests or secrets had to fear for his life, faced with a lawsuit and imprisonment and had to expect prison or torture. With such risks it was almost obvious and mandatory not to appear under the civil name. - But there were additional reasons why the true poet genius throughout his life was bearing false names, because
b) a single pen name (such as "Shake-speare") wouldn't have kept the "true" author sufficiently protected or secret. - „Fans“ gradually would have learned who wrote what, under which name, considering a gigantic literary output of the true "author", of a hitherto unkown extent.
c) multiple pen names must had have a business necessity based on such things as contracts with different publishers, etc. -
d) multiple pennames for divergent book genres (...having a different audience) have also been used as a matter of a sales point:
e) the author could use greater freedom of language by hiding under different names of translators
(e.g. Joshua Sylvester......
f) the author could choose names from the outset, for a novel playing in a foreign country or to spread transnational knowledge by using french (e.g. Abraham Fraunce, Everard Guilpin) , italian (e.g. John Florio, Guicciardin) or spanish (e.g. Aleman) author names
g) pseudonym improved the market chances of some works. E.g female gender domination in the genre belonging to the other sex, (e.g. Jane Anger, Elisabeth Grymestone, Rachel Speght, Ester Sowernam, Constantia Munda, Aemilia Lanier) or using only the initials or the initials of the first names so that the author's gender can not be identified
h) the author choose easy-to-relate pseudonyms or existing names of known deceased or living people that readers - as a recognition effect - might reference or remember more easily. (e.g. T.Overbury , R.Southwell, W.Shakespeare)
j) in order to avoid a conflict of "shame", writing putative "trifling", commonplace or „minor“ literature might like to prevent from learning with a pseudonym. ( ...such as Bartholomew Griffin, William Clarke, Nicholas Breton, William Basse, Richard Barnfield ,William Warner...etc.)..
K) many more