Some callings require that a member be worthy of a recommend. They do not necessarily have to actually hold a recommend. They just have to be worthy of one.
If a person was called to such a calling and then let the recommend expire, the priesthood leader would determine if the person was still worthy, and then proceed accordingly. The bishop would make that determination for a ward calling.
The bishop would advise the stake president in the case of a stake calling, and then the stake president would determine whether the person would continue in the stake calling.
Just because a recommend expires does not automatically mean a person has to be released from a calling. Bishops and stake presidents hold the keys to make those worthiness decisions.