Mark, perhaps the slab or foundation for the steps has shifted a bit and caused the mortar to work loose and therefore the bricks to become loose also.
Perhaps if some are loose, the probabality of all of them doing the same is very possible.
If you want, I would remove all the loose bricks and any others that will budge with a controlled nudge with a rubber mallot.
Get some vinyl or latex cement patch and go back and re mortar and set the bricks again. Allow plenty of curing time and rope the are off. Put plenty of mortar on the base of the step for good adhesion. But not too much for brick to be unlevel with remainder of step.