It's well documented (by me) somewhere... sorry I can't remember where. And too lazy to re-research and re-write.
Basically it's a combination of inside job (ex-employee(s) who needs a quick yen) and laughable IT safety process (really, NONE). Both have been remedied, supposedly.
Also I once again mention leaks caused a lot of pretty bad consequences for even casual fans. JAV stars have retired, new recruits are more difficult and increased regulation for the JAV industry are all blamed on the flood of leaks a few years ago.