What is Soft Fork?
In the case of the soft fork, the newer blocks get recognized and validated by the old software. Compared to hard fork, the soft fork does not require an agreement from all of the nodes to upgrade the software. It can be done by the majority of miners enforcing to upgrade the new rules.
When the miners accidentally violate the new consensus rules using the no upgraded nodes, a temporary divergence occurs in the blockchain resulting in occurrence of a soft fork. In this way, soft fork also occurs.