<<To use an example from a while ago(Optional bits in brackets):
Active - Robert cooks (the soup).
Middle - The soup cooks.
Passive - The soup is being cooked (by Robert).>>
To me it makes more sense to call it an anticausative. What a causative does is adds a causer to the verbs arguments. Most of the time, these verbs naturally lack a causer in their argument structure (i.e., "to sing" > "to make sing"), but since you can add as many arguments to a verb as you want, you can keep adding causatives till the lights go out (however, there is evidence in languages like Turkish where you can do this that, the more causatives you add, the more unacceptable the verb becomes).
Now, what an anticausative does is take a naturally causative verb (i.e., one whose argument structure requires a causer), and demotes that causer, so it becomes just like an intransitive verb with no causer, only since it's a naturally causative verb, you expect one. What I mean by that is, when someones says something like, "Nate is singing", the natural reaction isn't, "Well, who made him sing?" However, if a parent comes home and their child says, "The window broke", they just might be inclined to say, "Who broke it?" Anyway, you can't keep on adding anticausative affixes (if it was an affix) because, once you get down to one argument, you can't reduce it any longer. It would be interesting to see what a language that allows multiple objects and had anticausative affixes did with respect to this. But anyway, here are English examples that exemplify this:
Natural Non-caustive: Robert's afraid (of the soup?).
Causative: Robert frightens the soup.
*Anticausative: *The soup frightens. (However, interesting note: The room darkens.)
Natural Causative: Robert's cooking the soup.
Anticausative: The soup's cooking.
I think the reason that passives are so tied into anticausatives is because both demote something that's often the subject (passives, of course, always demote the subject; anticausatives demote the causer, which is often the subject), and raise the object (or causee). The difference is that most often in anticausative stuctures, the agent canNOT be reintroduced.
Passive: The soup's being cooked (by Robert).
Anticasausative: The soup's cooking (*by Robert, or ??because Robert's cooking it).
The easiest way to reintroduce the causer into anticausative expressions is just to say a new sentence.
So, that's my piece. I'll descend my soapbox now.