Why has Andy Coulson decided to resign today?

Why has Andy Coulson decided to resign today?
Is it because

a) It is a good day politically to bury bad news - Tony Blair is at the Iraq enquiry and speculation surrounding Alan Johnson's private life is rife.

or

b) After Sienna Miller allegations of phone hacking whilst Andy Coulson was boss at News of the World are more allegations about to appear, after all he has now resigned from two positions due to these unfounded allegations.

or

c) A combination of the two.