Its a negotiation that happens with the bank that is taking the company public. They are trading off the 'optics' of a bunch of people selling their stake at the IPO vs value. The lockup period for executive (officers) and non-officers is also negotiated.

The bankers want to strike a balance between employees (especially officers) who have concerns about being whipsawed with the stock price and the perception that they feel the company will grow in value over time so holding the stock is a "good" investment.

It's not an exception to the lockup, they are sold as part of the IPO. It's like taking money off the table in a VC round. Broadly speaking shares can come from three places when they are sold: new, which causes dilution, allocated but not granted, which is where the majority of employee grants come from, and finally vested ownership that is held by individuals. An IPO will have some mix of the latter two almost always. The relative proportion signals how bullish the insiders are to future prospects for the stock.