The most esthetically pleasing origin theory so far posted to this thread is poet's prarie-dog theory. Maybe it could account for longsword dancing, also.

A good source of what is known about the history of morris is Kieth Chandler, "Ribbons, Bells and Squeaking Fiddles"-The Social History of Morris Dancing in the English South Midlands, 1660-1900, Hisarlik Press, 1993.

An example of some of the interesting information found in this book: Chandler documents the displacement, between 1770 and 1870, of the pipe & tabor by the fiddle as the instrument of choice for morris accompaniment (Figure 9.1, a table of instrumentalists' birthdate-incidence by decade.)