I found this too.
Salt licks
Porcupines in search of salt sometimes encroach on human habitats, eating plywood cured with sodium nitrate,[2] certain paints, and tool handles, footwear, clothes and other items that have been coated in salty sweat. Porcupines are attracted to roads in areas where rock salt is used to melt ice and snow, and are known to gnaw on vehicle tires or wiring coated in road salt. Salt licks placed nearby can prevent porcupine damage.
Natural sources of salt consumed by porcupines include varieties of salt-rich plants (such as yellow water lily and aquatic liverwort), fresh animal bones, outer tree bark, mud in salt-rich soils, and objects impregnated with urine.[3]
If your toss pile has things that are salty, then that would be why.