Why is it in their interest to leak the announcement? Wouldn't that discourage other companies from filing through them in the future?

The leak builds up hype and investor curiosity about the IPO. It's like how visits by politicians to an area are 'leaked' so that the press is there to cover them.

You may be assuming that every employee of a company cares about the company's best interests. I find it unlikely that a janitor, intern, disgruntled employee, etc. would be unwilling to spill juicy details for a small fee or feeling of importance.

It's an intentional leak. Part of the "game".

If you send a press release, everyone thinks you're lying and boring.

If it's "leaked" it's sexy and ends up on TechCrunch as a "Report: Anonymous Source shows interesting financials"