How can fresh cavities form at the margins of sliver amalgam fillings on teeth?
Amalgam fillings are made from an alloy. The properties of the chemical elements bound into an alloy are different than the properties of those elements in different forms. The fillings do not release any biologically significant form or biologically significant amount of either silver or mercury. They are essentially inert.

Plus silver antibacterial effects are quite modest. The existence of fine cracks and gaps between amalgam and tooth provide wonderful hiding places for bacteria - and that shelter is probably more important to the bacteria than any negative effects of the silver.