At Tech Ed 2003, the presenter was asked this question, and the answer was that they wanted an irregular cycle to prevent it from occurring on a daily boundary (e.g. to distinguish from other daily tasks scheduled on the server / domain).
The site here(Link dead) speculated:
... (29 is the) first prime after 24, allowing it to have the least chance occurring in a regular pattern with any other server process; easing the investigation into problems
Another site appears to confirm this :
(Wade Hilmo) suggested 29 hours for the simple reason that it’s the smallest prime number over 24. He wanted a staggered and non-repeating pattern that doesn’t occur more frequently than once per day
