Enemies drop loot of different kinds, I found that level 3 - Azog the defiler the enemies will drop lots of loot, I.e - chicken sticks I had about 130 when I finished the level but as soon as you hit 99 the trophy will pop, it shows up in the top left hand side when you pick something up.
Edit - it has been brought to my attention that if you collect 97 of one item in one level then in the next level you pick up two more of the same item the trophy will pop then. Thanks to grampabig07 for additional info.