Should I be concerned?

[T-Tree]

Hi. I hope someone can help. 
Without giving tok much info, my garden has been a sanctuary for the last few years, after/during a difficult time. 

The trees in my garden have been healthy for a number of years. All of a sudden they appear to be looking unhappy. Can anyone tell me if this (attached) Is this part of a normal life cycle of a mature conifer or should I be concerned? If it is of concern, aside from disease, is it possible for foul play to cause this? The change has been rapid (weeks). There is a valid reason for this question. 
If a problem, is there a quick and effective way to stop the issue?

Thank you for reading!!

[Dan Maynard]

It'd be hard to rule out drought so I wouldn't panic.

[T-Tree]

Thanks Dan! We had a lotttt of rain here in July and August, so I had ruled that out… but maybe it is. The video shows just how much it’s shedding. It’s like conifer rain. 

[Logjam]

Appears to be normal dieback of the inner foliage, branch tips look to have good healthy new growth. They typically shed a lot of debris when branches are disturbed. 

[Mick Dempsey]

I’d say it’s toast, they die off very quickly once they decide enough is enough.

 

Very difficult to change the trajectory of Leylandii.

 

Wait and see.

[Tikay Musiiwa]

I don't think you should be very concerned. However, keep on eye in case it gets worse.