The reason behind Priyanka losing the film seems to be her leaving Rohan Sippy's film in a lurch. Priyanka was under the impression that she would be cast opposite Abhishek in that film until she realized that her co-star was South star Rana Daggubati. The actress then decided to drop out.
Rohan was appalled when Priyanka dropped out citing date issues and complained about this to Abhishek. Abhishek was also disgusted by her behaviour. And Abhishek is part of The Italian Job so he may have something to do with her ouster.
Already Abhishek and Priyanka weren't exactly sharing a rosy friendship. Priyanka's decision to drop Rohan's film may just have worsened matters. Priyanka's manager however, states that she hadn't even heard Rohan's script so she can't have possibly opted out of it.
Meanwhile, Priyanka may be replaced by Preity Zinta, but nothing has been announced as of yet. The film stars Abhishek Bachchan, Neil Nitin Mukesh, Kunal Kapoor, Akshaye Khanna and Katrina Kaif.