Not really an answer, more a set of loose thoughts fit only for a production with no budget for an evolutionary biologist.

In short: all humans are slightly different from us, here.

Evolution: epigenetics? Homo sapiens lupus having the potential, but never manifesting it under normal circumstances?
Let us not assume complete transformation into exactly a wolf or hybrid. Instead - aside from very plausible behavioral changes and body hair growth - something approximating it. Maybe even wider joints allowing a greater range of articulation, or sliding further down the length for instance, making the joint both look and act different. More likely, though, that it would not be a change, rather, a "direction" one could grow in. Also bear in mind that the existence of such creatures might also alter wolves - if you were born in those circumstances - your point of reference for a comparison would be different.
Now... why? Mimicry? temporary environment-triggered "paradigm shift" (lifestyle, energy allocation)? Blending into, or taking control of (easier when you're bigger than them) very widespread wolf packs? I am not qualified to answer beyond claiming it does not so far feel very implausible compared to some other extant phenomena.

Moon triggered: Might be same as with wolves howling at the moon - a mistaken myth. Or might be that the time is seen as optimal for hunting in that form - Homo sapiens lupus having inferior low-light vision to Canis lupus and other competing predators. Or syncing with some prey animals. Again, I can't speak much of plausibility.

Bite: pathogen or hormone, to trigger the dormant abilities, maybe actually only accidentally activating, or hijacking, the "normal" process for a more extreme effect, like with cannabinoid receptors in the brain - those weren't put there specifically for pot.
Whatever it is, it need not wholly take away the ability to sometimes look and act like a "normal" human, but the "similar, but more so" effect could give the affected "werewolves" a specialized advantage, leaving, in the end, only them, and a society of "pure" humans, never manifesting any of these traits... until directly exposed to the former, restoring the victim's lost ability to shift , and possibly permanently changing their instincts.

Not really an answer, more a set of loose thoughts fit only for a production with no budget for an evolutionary biologist.

In short: all humans are slightly different from us, here.

Evolution: epigenetics? Homo sapiens lupus having the potential, but never manifesting it under normal circumstances?
Let us not assume complete transformation into exactly a wolf or hybrid. Instead - aside from very plausible behavioral changes and body hair growth - something approximating it. Maybe even wider joints allowing a greater range of articulation, or sliding further down the length for instance, making the joint both look and act different. More likely, though, that it would not be a change, rather, a "direction" one could grow in. Also bear in mind that the existence of such creatures might also alter wolves - if you were born in those circumstances - your point of reference for a comparison would be different.
Now... why? Mimicry? temporary environment-triggered "paradigm shift" (lifestyle, energy allocation)? Blending into, or taking control of (easier when you're bigger than them) very widespread wolf packs? I am not qualified to answer beyond claiming it does not so far feel very implausible compared to some other extant phenomena.

Moon triggered: Might be same as with wolves howling at the moon - a mistaken myth. Or might be that the time is seen as optimal for hunting in that form - Homo sapiens lupus having inferior low-light vision to Canis lupus and other competing predators. Or syncing with some prey animals. Again, I can't speak much of plausibility.

Bite: pathogen or hormone, to trigger the dormant abilities, maybe actually only accidentally activating, or hijacking, the "normal" process for a more extreme effect, like with cannabinoid receptors in the brain - those weren't put there specifically for pot.
Whatever it is, it need not wholly take away the ability to sometimes look and act like a "normal" human, but the "similar, but more so" effect could give the affected "werewolves" a specialized advantage, leaving, in the end, only them, and a society of "pure" humans, never manifesting any of these traits... until directly exposed to the former, restoring the victim's lost ability to shift , and possibly permanently changing their instincts.