Termination notices can be issued for a variety of reasons:

  End of the fixed term of the agreement
  Breach of the agreement
  Non-payment of rent
  Sale of the property
  No grounds

Each of the above reasons carries its own notice period. 
If you receive a termination notice, the landlord / agent should attach a covering letter detailing the reason for the notice and what you are required to do.
If you have any questions, telephone your landlord / agent straight away.