They could be ready and this is the next logical step. Earlier investors could be pushing for liquidity. Maybe some of the early people - especially founders - want to move on and do the next thing.
On the last point, non-trivial share holders are generally locked up from selling for 3-6 months after the IPO to reduce the likelihood of a "pump & dump" scheme.
They may also be hoping to ride PayPal's rich valuation.
With investors awarding such extreme valuations to Facebook, Netflix, Amazon, Twitter, PayPal, LinkedIn - Square might just think they can land a $10+ billion public value.
I like the conspiracy theory that Square filed to IPO in hopes of forcing a quick acquisition by Google / Apple / Amazon / Paypal so that Jack could go take over the CEO position at Twitter. Clearly unlikely but the intrigue is fun.
