The LA Times:

A source who is unauthorized to discuss the case told The Times the FBI was provided with the ability to incorrectly guess more than 10 passwords without permanently rendering the phone’s data inaccessible. That allowed the agency to use software to run through potential pass codes until it landed on the correct one.

Given alternatives, like bypassing the password process entirely, or breaking the encryption, this seems the most obvious path.

Also from the article:

Last year, an Italian company that bought and sold bugs saw its entire database leaked onto the Internet. The security issue could explain why the FBI and the outside party are being so secretive about the process.

And:

Apple generally doesn’t reward bug-finders with cash. But given the publicity in this instance, experts said Apple could turn to the black market too.

If the FBI keeps this process secret from Apple they will, ultimately, be sharing it with the black market.

Of course, there’s always the possibility that the approach used no longer works on more modern phones, more recent releases of iOS. If that’s the case, Apple will certainly be seeing more All Writs motions from the FBI.