speculative answers
1) What motivation would Khamenei have for rigging the elections? He's worked with (and controlled) moderate reformers (Khatami) before, and Moussavi wasn't a dangerous radical, but a full member of the establishment.
growing power of Ahmadinejad, for one.
also, ironically, worried about legitimacy. feels he is in a weaker position than he was when Khatami was elected. as you feel power slipping away you grasp it ever tighter. rigging the elections so crudely an "overreaction" as Vim mentioned.
2) Ahmadinejad leaves Iran almost as soon as the result is called, and has said almost nothing of substance about it so far. Why?
if you want to downplay something don't talk about it.
3) Where is Khamenei? He has said nothing at all... If he is in fact the target of an attempted coup (rather than one of the instigators of it) Ahmadinejad's position suddenly starts to look at lot weaker.
what if the coup is by Ahmadinejad & various IRGC etc supporters?
4) I don't think the IRGC anticipated this level of protest, and I think events could be overtaking the Ahmadinejad faction...
they probably didn't anticipate this but I'm dubious about "overtaking". listen, dude, these regimes the world over & their hombres con armas - I very unenthusiastically tell you that they will murder & disappear & torture a great deal before they let themselves be "overtaken"...