It's not without good reason that the authors chose to say
"Your password will expire tomorrow".
The situation is conditional: the password is in a state that is set to change "tomorrow" (unless you do something today.) See the next sentence to understand the significance of the modal verb indicating the "default" behavior as it stands.
will (ODO)
5 Expressing habitual behaviour: They will do this at a certain time of day …
However, the situation can be changed.
The next sentence begins
To change your password …
OTOH,
"Your password expires tomorrow."
is a plain and simple statement of reality without an indication of possible alternative. (You can't do anything about it – no matter what you do, it expires – For your information only … )
The user needs to be alerted and prompted for action: ... will expire ... ("is going to expire") has the tone of an impending situation that can be averted.
Unconditional statement:
A typical dog lives for 13 years, so it adds up to quite a lot of cash. (src: the Web)
Conditional:
A typical dog will live for over twelve years … Just be sure that (i.e., if --ed.) you can give the right amount of love and care (src: the Web)