Why do my new isa brown hens aggressive before bedtime?

I purchased 2 isa brown hens 2 weeks ago. They are currently 18weeks old as advised by the pet store keeper. I let them out free ranging an hour before sunset. They are friendly, timid, curious while I'm walking around the garden, inspect each corner and run away if I try to pat them. Last week they would walk themselves to bed when the sun sets. But the past few days one has started to get aggressive 10 minutes before bedtime, run towards humans standing around, threaten to jump and has jumped and fly kicked by father's back. We have stood our ground and held it down for a minute to calm it down. This is starting to become a regular daily bedtime crazy snap this week.
Why is this happening and how do I stop this bedtime behavior?

 

Here they are while in their coop. The one acting up is the one sitting down.
Strangely they are normally quite shy and runs away if we get close. It's just the 10minutes after sunset when she gets nasty.

 

I spoke to my work buddy who owns 36 chooks about this and he said that its probably cause its dark and they think I'm a predator because they can't recognise me.

I have read about bad night vision for chooks.
I think it's true because the past 2 days we sat at least 6 metres away from them after sunset. They are still more edgy than when the sun was up, but now put themselves to bed without the angst and fly kicks.

Thanks for all your help. You guys make this such a great forum.