I would think that they see something that startles them. Even a flying bug, or a shadow can make mine jump. As they get older they learn that shadows aren't going to kill them, but when they are young, they will jump at everything.
50 and 60 is actually nice summer weather for us.
I have my about six week old chicks in that kind of weather right now, they are doing great.
Maybe you can set up a little playpen for them. Perhaps set up a roll of wire to temporarily enclose them while you are working right there.