See this possibly relevant NTSB report:
http://libraryonline.erau.edu/online...s/SIR96-03.pdf
Apparently similar accidents are well documented in the Robbie accident/safety literature. Experienced instructor flying with a low time student. Student makes a large, abrupt cyclic input for some reason, MR flaps down and chops off tail boom faster than instructor can respond. May not be what happened in this case, but apparent similarities to many documented occurrences.