Few passengers.
Northern is a very diverse network. Some villages served by Northern have no other public transport meaning someone without a car needs to use the train to get to the nearest supermarket or doctor's surgery. Some routes have relatively low loadings all day, others have relatively high loadings all day even in the counter-flow direction at peak times and during the middle of the off-peak period.
I've always found the London commuter network to be carting long trains with just a handful of passengers in one direction (probably some of the emptiest in the entire country) and then the same train gets a reasonable or high loading when it later works a service in the opposite direction.