My Hoover fridge-freezer has suddenly started working funny.

The top fridge is work fine and is actually quite cool but the bottom freezer keeps turning on and off by itself. Basically I noticed that it reaches freezing temperature then shuts itself to defrost mode.

The freezer is frost free whilst the fridge is not (if that can be of any help).

What could it be?

Any suggestion would highly appreciated.

Many thanks.