Why Pettirgrew hid for twelve years

'What, scared to hear your old master's name?' said Black. 'I don't blame you, Peter. His lot aren't very happy with you,, are they?'
'You haven't been hiding from me for twelve years. You've been hiding from Voldemort's old supporters. I heard things in Azkaban, Peter... they all think you're dead, or you'd have to answer to them... I've heard them screaming all sorts of things in their sleep. Sounds like they think the double-crosser double-crossed them. Voldemort went to the Potters' on your information ... and Voldemort met his downfall there. And not all Voldemort's supporters ended up in Azkaban, did they? There are still plenty out here, biding their time, pretending they've seen the error of their ways... If they ever got wind that you were still alive, Peter -'
'Don't know... what you're talking about...'  said Pettigrew again, more shrilly than ever.
'I must admit, Peter, I have difficulty in understanding why an innocent man would want to spend twelve years as a rat.'
'Innocent, but scared!' squealed Pettigrew. 'If Voldemort's supporters were after me, it was because I put one of their best men in Azkaban - the spy, Sirius Black!'

THE SERVANT OF LORD VOLDEMORT - Pg 391