For what reasons might a full professor lose tenure?
The details are likely institution (and or state) specific. 

From the University of New Mexico Faculty Handbook one sees that the options there are:


"Adequate Cause", including academic incompetence, neglect of duty, serious violation of policy, serious crime, loss of license (medical), or other serious deficiency.
Financial emergency - that is, the university runs out of money to pay them.
Termination of the program/department.
Health reasons (permanent disability with no reasonable accommodation possible).