Senior administrators often have backup positions, something that doesn't happen usually in the private sector, as Owen notes. Stephen Karlson, however, puts the matter in some context.

Traditionally, deans, provosts, and chancellors (using the Wisconsin terminology) are senior members of the faculty given additional responsibilities that they might hold by virtue of their long experience and wisdom ... who might hold such positions for a few years before returning to faculty.

...mid-level university administrators (holding positions other than department chairman, dean, provost, chancellor) sometimes are failed scholars, frequently from disciplines with large reserve armies of the underemployed, and sometimes hired for purposes that may or may not be central to the university's mission. Whether those mid-levels ought to be accorded the same protections as accomplished faculty (who sometimes deserve it; there is a reason one traditionally greets the newly seated chairman or dean with condolences) is another matter.

When one is hiring someone who has tenure, it is customary to ascertain whether the person would warrant tenure at your institution -- and if not, why hire him or her? -- and then grant it on arrival or after a usually perfunctory review a year or two later. This frequently happens for faculty, and there's no reason why it wouldn't happen as you took a dean who was tenured in his or her university to be a dean in yours. Unfortunately, the state system in Minnesota does not permit us to tenure an outside dean. Thus the ones we get are either insiders -- who deserve either condolences or condemnation for masochism, or those who have decided that administration is more rewarding than teaching and research -- or outsiders who most likely were failures elsewhere (otherwise, why go from a job with tenure to one without?)

Another point, however: A dean or a chairman has additional power in the system by dint of having a retreat home. Senior administrators come and go, and tenured faculty who become deans and chairmen usually survive them. They have a greater stake in the institution, and also the ability to oppose itinerant careerists who want to impose, say, mandatory diversity training to feather their resumes for their next posting. Firing me as a chairman for being a royal pain in the tuckus isn't much help to the president. I go down the hall and open up a second blog like SCSUPRESIDENTMUSTGO.blogspot... or something.

It is with great hope that I note I have only two more years in my term as a department chairman.