In 2013 the White House Office of Science & Technology Policy (OSTP) released a memo directing federal agencies (those with more than $100M in R&D expenditures) to make both articles and data supported by federally funded research freely available to the public within one year of publication (explanation from NCSU web site).
So, if your research is funded by an external granting agency or organization, or one of your coauthors is employed by a U.S. federal agency, you may have requirements to provide a data management plan document or a copy of your data for future use by the agency, organization, other researchers, or members of the public.
Below are resources that will help you determine what requirements your funding or coauthors may have.
Information in this space changes rapidly. Please verify that the requirements linked are the most current before submitting your final data management plan.