My DH is in a flap. He found out today that due to an increase in pension contributions following his pay rise in September, he may end up with a negative sum and there owe his trust money. How can this be correct?? He's at the bottom of band 5 (it will affect band 3/5/8a apparently), and is due to moving up to the next pension threshold and due to workers being paid back pay from April. His Trust have mentioned helping staff with plans on how to pay money back that it owed 
It makes no sense to me, but is obviously worrying, as this is not the time for us to have less coming in to the household,
Can anyone explain this to me?