Do you have wild turkey around? I've seen them engage in such behavior.
Nope, no birds. Earlier I had problems with birds ripping them out of the ground, so I put some nitting over them.It is very light duty, but does seem to keep the birds out. Also we had a grasshopper infestation, but I seem to have been able to manage them, They would a least eat the leaves. I just can't figure out what would bite the leaves off without eating them. Vandal beetles, maybe? :^)
I had a deer eat the stops off all of my sunflowers this year. The deer did not eat the leaves of the sunflower but made quite a meal from the hosta leaves and okra plants.