Sometimes you may find that a Task you completed in the past, perhaps even long ago, has magically resurfaced and found its way on to your calendar. Magic can be wonderful, but it's usually not very fun for your calendar to be mysteriously changing around you.
Here are two common causes of this phenomena:
You have (perhaps unintentionally) activated a repeating Google Task. Read more about repeating Google Tasks and how to ensure they don't keep creating clones in the future.
You have (perhaps unintentionally) deleted a historical event associated with that Reclaim Task. When you do that, Reclaim will take that as a sign that you didn't do the work at that time and will reactivate the task to be scheduled again in the near future.