I believe he was 1 of 4 co-founders, and then raised 11 (according to crunch base) financing rounds. If you get diluted even a very reasonable amount 11 times, you won't end up with a large percentage of the company.
Basically if a company is successful they'll probably raise many rounds and then end up with their founder/ceo only having a small portion of equity. At this point, the founder/ceo might be rich and has "made it" so could easily leave and be happy, but investors may want them to stick around. One way to get them to stick around might be to compensate them with even more stock.
Obviously depends on the situation, but it seems silly to lose a founder/ceo because investors didn't want to dilute their equity and give the founder/ceo a little more.
