I disagree that it is implausible; I find it quite likely (I just deleted a comment that mirrored gburt's).
That it happens when you need to throw a punch is not evidence of intent, as that is a pre-determined sequence. There are likely only so many sources of random-looking behavior available, so it's quite reasonable that a background character's movement would be tied to the same source as the source that determines how often the boss is vulnerable.
edit: To clarify, the two events are clearly linked, but that does not mean they were intended to be linked. I am in agreement with Zikes' reply.
That sort of double-duty was pretty common in old console games, which were limited in both memory and available compute time, so that's a pretty plausible theory.
But seeing as one of the game's creators has mentioned they liked to put in contextual cues for when things were about to happen, it also seems pretty plausible that it's intentional.
The entire game is based around timing your punches, and the whole thing is filled with subtle timing cues that, once you learn them, make it possible to win every time. I find it much more likely than not that this was intentional, given how many other intentional cues exist.
That it happens when you need to throw a punch is not evidence of intent, as that is a pre-determined sequence. There are likely only so many sources of random-looking behavior available, so it's quite reasonable that a background character's movement would be tied to the same source as the source that determines how often the boss is vulnerable.
edit: To clarify, the two events are clearly linked, but that does not mean they were intended to be linked. I am in agreement with Zikes' reply.