תמים תהי' עם ה' אלוקך
Frum Yidden trust in Hashem & have no fear of witchcraft
The Sefer שלחן העזר quotes the סהמ"ט* (I don't know the Roshei Teivos- Please help) "Chasanim & Kallos before walking down to the Chupa untie all the knots they are wearing.
The reason for doing so is to prevent from כשפים (witchcraft) which might put a spell on the couple and prevent them from having relations.
Another option of making this witchcraft ineffective is by placing a piece of gold or a gold coin in the Chosson's pocket.
The above is not a Minhag but a Segula people adopted to prevent witchcraft from harming them.
The יראים & תמימים (who fear & trust Hashem) rely on תמים תהי' עם ה' אלוקך & שומר מצוה לא ידע דבר רע (Don't bother unknotting)"
*I suspect this Sefer to be the Mokor (origin) of this Minhag