The rationale was that if, password hashes got compromised, the attacker would only have until the next forced rotation to crack the passwords and take over accounts.

edit: or, in particularly terrible systems, if plaintext passwords were leaked.

Of course, that's only useful if it doesn't affect any other password security concerns, and it turns out that users who are forced to change their passwords frequently pick worse passwords, which is a bigger problem than the scenario this was supposed to protect against.