Why are these pictures automatically transferring to my phone?

Today, for the first time ever I found myself completely lost as to how to fix what is probably a simple technical issue. All I was doing was transferring pictures from an old MAC HD that had been removed was in a sata to usb adapter to a newer toshiba external hard drive. While this was happening my sisters phone started receiving every singe picture that was transferring. What is happening, we both have galaxy S5, I was not using any cloud software during the transfer just dragging a dropping files. These are just basic external hard drives. My sisters phone has never been hooked up to my computer, I really don't understand what is happening here. The pictures are not uploading on any other phone.

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