Never happened to me when tying but worst it has when fishing. I learned to check my hook when several fish get off. Tightening hooks too much in the vice can cause breaking. Do you use the vise to flatten the barb? The picture looks as if the hook broke at the barb. Fly hooks unlike common hooks are hardened and tempered to prevent straightning Be that as it may, the hooks should have been tempered enough not to break easily. You probably got some bad hooks.