According to the FAQ, Turbine became aware of a compromise in the forum database that would allow unauthorized access from outsiders. The company took the forums offline, brought in security experts, and fixed the bug that caused this issue. Turbine also claims it has strengthened its web security and that no payment details, including credit card information, were in danger of being stolen.
As part of an effort to make sure all players were secure, Turbine sent out notices to a few customers with particularly vulnerable passwords. In the email, the players were informed that their passwords were reset for their own safety.
The FAQ ends with a few Dos and Don'ts about password creation which all MMO players would be wise to adopt.