Failed Foundations can occur in buildings for many reasons but fundamentally because the sub-soil below cannot support the foundations that ultimately take the loading from the building above.
In most cases, such a failure in London is because the clay sub-soil shrinks, due to the dry winter and summer seasons because of climatic changes. In some cases, the construction itself is inadequate and the failure is due to insufficient foundation depths or founded on a sub-soil incapable to supporting the loading.