The 487 Response indicates that the previous request was terminated by user/application action. The most common occurrence is when the CANCEL happens as explained above. But it is also not limited to CANCEL. There are other cases where such responses can be relevant. So it depends on where you are seeing this behavior and whether its a user or application action that caused it.
15.1.2 UAS Behavior==> BYE Handling in RFC 3261
The UAS MUST still respond to any pending requests received for that dialog. It is RECOMMENDED that a 487 (Request Terminated) response be generated to those pending requests.