I have ice forming on the copper lines where they connect to the furnace ducts, what causes this?

I have ice forming on the copper lines where they connect to the furnace ducts, what causes this?

Them being cold because your airconditioning is running.. and there being enough humidity for it to form.

Question here is how MUCH ice are we talking... if its HUGE chunk... you might need to have an HVAC guy check your system out because you may have a low refrigerant charge and its run time vs off time is way too long as this would normally melt off during the off time.