The customs inspector probably tagged one or more of your items as prohibited for import, in which case they deny entry. They alert UPS, UPS will then contact the sender to see if they'll agree to pay to have the item shipped back... I guess UNC agreed.
Seizure and destruction happens when the sender disagrees to take the item back, or the mail carrier refuses to continue handling the parcel (which would've been the case had it been guns, drugs, dangerous chemicals, etc.)
You can take a refund from UNC (minus re-stocking fee) once it returns back to them, or pay to have it reshipped and hope for better luck..
May I ask what were the exact contents?