I have 5 amano shrimp who systematically decide they are going to kill a plant. I was told originally it was because there wasn't enough food and they must be hungry but i have seen food laying almost ontop of them and them still going for the plant. Mine over the year have eaten every piece of wisteria I had in my tank. I even chased them off one plant and placed pellets down for them, moving the plant to the far end of the tank, they left the pellets and went straight to the plant and destroyed it.
They have also destoyed cabomba simply shredding it until I had only stems left. I dont know why they do it, they are not hungry, have grown huge, are berried quite often (sadly) so must be happy enough. I think its just something they like to do every now and again.