The Housing Act (1988) sets out much of the law behind today's Private Rented Sector. One section of it, called Section 21, describes a process by which landlords can evict tenants.
To evict the tenants, they can serve a standardised notice form. Section 21 describes the circumstances under which the notice can be served, but the main criteria is that the tenancy is not within a fixed term. That is, the tenancy must be periodic.
Crucially, no grounds must be shown to exist for a landlord to use Section 21. The tenant can be paying their rent, looking after the property and meeting all their responsibilities as tenants. Alternatively, they could be flouting them. It doesn't matter. The landlord does not need to give any reason for the eviction. The landlord does not, for example, need to prove that the tenant is in arrears. They can simply serve the notice to begin the eviction proceedings.
For this reason, Section 21 evictions are sometimes called 'no fault' evictions, although many landlords argue that they are usually used to evict tenants who have been breaking their contracts terms, and so calling them 'no fault' is misleading.
More information on Section 21 evictions: