To be fair to them, it's very common, in all parts of the country, for passengers to travel on trains where there are no staff in that part other than the driver, because the guard is in the rear part. It's also common for passengers to travel in parts of the train where there's neither driver nor guard, nor any other member of staff, such as in the centre of a trio of 170s.Not odd or lazy, it's due to lack of staff, a set can't convey passengers without a staff member on that part of the train, frustrating for passengers though.
You say 'no apparent reason', things aren't done just for the sake of it!