There have been some reports in the past of waitlist movement before May 1, probably because the college in question was getting a high enough rate of declined offers early that it predicted before May 1 that it would underyield from its initial admit batch. (Analogy: an NBA game at the end of the third quarter has a 40-point difference in scores. Even though the game is not over, the result is highly predictable, and in-game decisions based on that will be made.)
The effect of any individual admit contributing to this kind of thing can vary. An admit declining an offer at a reach school (which expects a large percentage of such admits to matriculate) probably has more of this kind of effect than an admit declining an offer at a safety school (which expects most such admits to decline).