It could be for a number of reasons. Firstly, if you are taking on additional work for a company, as a temporary employee, and you will still be working for your permanent employer you will have to pay basic rate tax as your tax free allowance can only be used against a single income.
If the part-time work will be on a freelance basis you will need to register as self-employed. If the company you will be working for will not deal with a sole trader you will need to use the services of an umbrella company or form a limited company.
