Chickens are prey animals. From the looks of your brooder, you need to reach in over the top to pick them up. They may see that as threatening (think aerial predator). Because chickens are prey animals, they are naturally wary of anything trying to catch them.Should I still try and pick them up and hold them or just leave them be?
How old are they? They look a little big for the brooder already. Crowding can cause aggressive behaviors toward other birds.