As the previous poster said they are simply applying the debt to both of you, making you both liable. While its their choice to do it you don't have to accept it at all, go to another lender and explain to them why you are changing.

Additionally if the property is in your name I recommend a disclosure agreement where the other resident a knowledges they have no claim on the property for any reason unless you specifically say, otherwise a split up etc could give them 25-50% if not more of the value. You should always be prepared for the worst when dealing with property investment , I know its not a nice thing to talk or think about but its still a fact of life.