Hello David
This is a good question, and leads us into a deeper understanding of how fiber links with SFPs work. A fiber connection using an SFP transceiver (or any type of transceiver for that matter), typically uses two fiber strands, one for Tx and one for Rx. This arrangement can increase the probability of a unidirectional link in the event that one of the two fiber strands is cut or fails in some way.
In the past, the operation of the Tx and Rx of such a link was quite independent, and if one fiber failed, the other would continue to function, greatly increasing the probability of a unidirectional link failure resulting in an L2 loop.
Now most modern transceivers have mechanisms in place that can detect such a failure and will consider the link down even if one of the two strands is still operational. But even such mechanisms may take several seconds to kick in due to the nature of fiber optics, and there may be a period of time where the link still appears to be up/up even though one fiber has failed.
It is for these types of scenarios that LoopGuard and UDLD are needed and should be in place, especially in high-speed mission-critical core networks. Does that make sense?
I hope this has been helpful!
Laz