In landlord and tenant law, if an Assured Shorthold Tenancy initial period expires without either party giving notice, it automatically becomes a statutory periodic tenancy running for the same period as before - so in your case it is now a month to month tenancy. This may be fine for the mortgage company or they may prefer you to always have a fixed term in place. However periodic can appeal more to tenants if they don't want to commit to another 6 months or a year.
You have done nothing wrong, it may just not be in accordance with your mortgage company's preferred practice.
This leaflet is quite useful for explaining the difference
http://www.communitie...ousing/pdf/138289.pdf