MBP crashed to complete shutdown except backlit

My MBP 15.4", 2014 Mid on Yosemite 10.10.5.

Just found out that when connected to an external monitor via HDMI and streaming video Full screen on built-in retina screen using Safari (version 9.0.3) on a website with JW Player (version 7.1.0), the whole MBP would crash to complete shutdown leaving only the backlit of retina on. This happened all the time on this website in just a few minutes of streaming. I have to press Power button for a few seconds to reboot MBP. However this would not happen if there is no external monitor or the streaming video is playing on the external monitor.

In my limited findings, I believed this problem is not related to whether on AC power or battery. I also ruled out overheating issue. I reported this problem on Apple feedback page and hoped someone would in contact with me soon.

In the meantime, I may do more tests to replicate this problem on monitor with display port or so...

Thanks

Guruduck