Not terribly uncommon in nonprofits either -- Often times they're on overlapping terms (3/4/5 year schedules) that occasionally coincide to term-off half the board at once. That's another difference I've noticed actually, many nonprofits have specific terms for their BOD whereas most Corps I've seen have open-ended commitments.

It's common in nonprofits if they wish to change what flavor of board they have. It looks to me like Tor is changing from an executive board to a single executive and an advisory board. This is always a risky move, but very plausibly the right move for Tor right now---and certainly essential some time in the next few years.