If it's an entire case assortment that's shelfwarming then it's a combination of ignorance on behalf of the buyers/suppliers for individual chains and bad luck. That is: the retail chains don't know enough to realise which waves or Transformer franchises are 'good' and 'bad', so they stock far too many of waves with toys that will inevitably shelfwarm. And, because of this ignorance, it means it's mainly bad luck that results in shelf and pegwarmers.

If it's just one toy from a case assortment shelfwarming then it's Hasbro's fault for making a crap toy or shortpacking the wrong items. If they shortpack awesome toys and longpack a crap shelfwarmer then they should have known better. Also, sometimes a whole case assortment is just multiples of one crap toy, like Transmetal Scavenger or Recon Ravage and again Hasbro is to blame for shelf/pegwarming.