My 2 cents, with no inside information:

1) They're losing a ton of money. They barely have any revenue, and have over 1,000 employees, many of which are high paid engineers. They also have large technical overhead costs. My guess is they want to sock away 3-4 years of cash since they don't want to go back to the market too often.

2) M&A. Perhaps they buy some technology companies. Or Twitter? Or who knows?

3) They may be cashing out certain earlier investors.

When companies IPO, they have to release a significant amount of equity no matter what.