Power Adapter temporary turns off when plugging in USB

I have noticed my power adapter temporarily (3-5 sec) turns off (green light goes off and computer switches to battery) when I plug in a USB hard drive to the port near the power port. Specifically, I notice it when I plug in hard drives that are have their own power and ones that draw power from the USB port.

Its not really a problem in my view, but I want to make sure its not a sign of a more serious issue. Has anyone else experienced this?

Thanks!