Saturation, more pins that have high demand (AFMR, GB, etc) vs less-demand pins (Melons, Aerosmith, etc), more boutique pins out/coming out, anticipation of possible future remake titles (MB), anticipation of SW pin, etc etc. It'd be quite a long list if you simply made a list of all titles released in the last 12 months coupled with all future titles announced.
Just a lot of stuff at the same time driving prices down.