When a task is assigned a user but they aren't also explicitly subscribed to it, any change to the task using the "Edit Task" window will also add them as a subscriber.
steps to reproduce:
- create a new task, assign to user X but do not add X as subscriber
- click "Edit Task" to open editor but make no changes. note that X is not listed under "Subscribers".
- click "Save Changes"
- note task now has X as subscriber. the task's timeline shows "added a subscriber: X"