It has been known for rear pads on the MINI to wear quicker than the front ones. I think this is possibly due to sticking callipers, which does happen.
I know mine did this. My dealer, who had serviced the car only months before my rear pads wore down to the metal backing, said that they don't inform customers when their pads are low at service because everyone drives differently.
The lesson is not to rely on the brake pad wear indicator and check yourself. I say this because I thought my car had wear indicators and I found out too late that it doesn't.
Oli