a couple three reasons I think, but not sure
1. you sign up for a ready go, but don't play it immediately, and it fills up pretty fast, the 12 hour clock starts(assuming you actually get 12 hours) and you don't make it back in time to actually play. My guess is this would show as a withdraw, maybe not, but just throwing out some scenarios. (i have had this come close to happening 2 times now in last two days)
2. Similar to #1. You sign up and start the match and life calls and you don't make it back in time to finish the round.
3. In some of the open non tier based RGs a lot of the top players know that in order to make the top 30 and make even just a little bit of credits, they have to hit certain scores. If they come out and on the first few holes just play like crap and make some bogeys or whatever, they know there is no chance of making anything, so maybe they withdraw. I would think they still finish the round for the experience or whatever, but maybe not. Maybe they just play so many that winning the credits is more important and if they know they can't they just withdraw and move on to the next RG.
I'm sure there are other reasons, just my thoughts