Lucifer
But with respect, that's like the excuse for a delayed departure, "due to the late arrival of the operating aircraft." Yes, it's true, as far as it goes, but it doesn't really provide much enlightenment.
bmi could have chosen to sign new leases and keep the aircraft (most lessors would be happy to leave their aircraft with the existing operator rather than incur redelivery costs and downtime). Clearly they chose not to do so, and to return the aircraft at the end of their leases instead - because they felt the A321 was too big for most of their operations, because the lease rates they were being quoted were out of line, who knows?