If you think back it was 13th September 2001 that Ansett flew for the last time
Very soon after that Virgin Blue scoured the world to get any aircraft it could. I recall the 737-700s were the most 'available' at the time. So if those aircraft were leased for 10 years, they'd be coming due around now and in the next 6-12 months.
Part of the strategy may be to return these leased aircraft and take new ones from the factory line. It appears most of these new ones are 800s. This could be part of the order Jayden mentioned.