A soft fork requires the majority of miners or users to agree to enforcing the new rules of the soft fork.
Discussion
I think that if a small minority of users activated the fork, and a block that violated the new rules was confirmed, the minority would either reject it and a hard for would occur, or they could be stolen from.
If 51% of miners activated the fork, they could successfully reject and reorg any blocks that break the new rules.
If less than 51% of the miners accept it, it becomes a hard fork.
I might not have this all correct, but this is where my thinking is.
In none of the cases described does a hard fork occur. It is a soft fork despite the forking of the chain. Both chains are valid Bitcoin blockchains but the majority of nodes are following the one that contains more work.
I suppose you then end up with some form of an alt coin, which could become Bitcoin in the future if the chain contained more work.