iPhone disconnects from mac?

So, I've been working on some phone automation with my macbook, when a number of iPhones are connected (with a usb hub, all to the same port) - after a while I notice some of the phones still get power but are no longer recognized as connected to the macbook (That is - they wouldn't appear in the list of connected devices until I reconnect them).

That causes my Appium scripts to just crash and stop working until I manually disconnect and reconnect the phones.

Anyone has any idea what is causing this?