Rumors abound. The most prominent rumor is that it was because of a bad public reaction to a Jesse Jackson AMA. Some leaked info from a supposed disgruntled Reddit admin says it's because Ellen Pao pressured Victoria into pushing an NDA + promise of a cut of future sales resulting from AMAs on the AMA subjects, which Victoria disagreed with a little too loudly.
Sorry, I was a little vague. The mods of r/IAmA have been in contact with reddit admins about the situation, and from how the timeline was explained to them by reddit staff, the decision was made before the Jesse Jackson AMA even happened.
