external backup runs after ejecting
I'm using a mid-2015 MacBook Pro running MacOS 10.14.6. Approximately once a week I run TimeMachine to back-up two external portable hard drives (made by different manufacturers). About 50% of the time after I eject the drives upon completion of back-up, the drives will spin down, then immediately spin up again. At that point I don't have much choice but to either pull the USB cable while the recently used drive is spinning or shut down and then pull the cable. Either way, this seems like a hassle and I don't want to damage the external drives somehow.. Any ideas as to what I can do to get the drives to eject and stay "off" instead of spinning back up again? I've run Disk Utility numerous times and have tried resetting NVRAM/PRAM with no effect noticeable. I've restarted before backing up as well.
Thanks in advance for your help.
MacBook Pro Retina