"It is possible that someone stole your tefillin and sold them to a Jew. If you aren’t mafkir your tefillin, a Jew might be wearing your tefillin thinking he is doing a mitzvah, but really be doing the aveirah of using stolen tefillin." (source)

"Rabbi Weissmandel was a Godol with limitless Ahavas Yisroel, and he was Mafkir himself for the sake of saving Jews, so please read what he writes before you utter another word." (source)