Spreader, why do they not flop back or forward?

[Kayakeur]

What keeps a spreader from flopping forward or back?(toward bow or stern?)
Also where each spreader connects to mast is that supposed to swivel?
Thanks. (trailer sailer)

[Spot]

On my trailer boat the spreaders rotate and pivot and are held in position when the upper shrouds are tightened as the mast is raised and the forestay is attached. I have lower shrouds as well but do not have a back stay. I have to watch that the spreaders are located within reason before stepping the mast. In the enclosed picture, the spreader and tang for the lower shroud are rotated, in use they would be about 90 degrees anticlockwise so that the slot for shroud is chasing the shroud and the tang would point back to the chain plate on deck. The other pictures show the rig in place.

As an aside, running a flag up a halyard usually results in me fouling something so it is good that the mast get un-stepped as part of the process...