I don't think it's that common for equity to be massively cashed out (to the tune of nearly $1b) before the IPO. Shows a pretty strong lack of confidence in not only the long-term viability of the business, but even its medium-term viability, or else the principals would be content with the normal practice of slowly cashing out with steady monthly post-IPO stock sales. As far as I can find, Google principals didn't significantly cash out before their IPO.