It's not too common, but larger amounts aren't unprecedented: two big ones that had even larger ownership were Microsoft (Gates still held 49% of Microsoft at the time of IPO) and Amazon (Bezos & family held 52%, with Bezos himself holding 42%). The Google founders also owned a combined 32% (16% each). Those companies were rather more successful than Box, though.
I think that's generally the rule. The more successful the startup, the larger the stake the founder maintains due to being in a better position in regards to VC rounds.
The only VC with an investment in Microsoft was David Marquardt's Technology Venture Investors. At the IPO, TVI owned 6.1% of Microsoft.
The remaining 93.9% belonged to the founders, the Board, the employees, and friends and family. Everyone listed on the S-1, with one exception, took a bit off the top but held on to most of their shares.
The one exception was Bill Gates's sister's trust fund, which owned 20,000 shares -- 0.1% of the company. She could've been a hundred-millionaire if she'd held on to it, but her trustee sold it for $400,000 and moved on.
Microsoft had so much cash lying around they were using t-bills (or some other kind of bond) as bookmarks (don't remember which Gates bio I read that in -- Hard Drive maybe). When Microsoft IPOed it was basically owned by its founders. It never wanted for cash.
