It looks like this is part of a normal "exercise and sell all your options as soon as you receive them" plan. Notice that on 5/11, he goes from 70,143 shares to 0 over the course of the day. And then again on 5/12, he goes from 70,143 shares to 0 over the course of the day. It wouldn't have gone back up to 70,143 the next day if he'd just sold his lump sum of founder shares.