Only reason for a drive to spin back up is that a read is currently happening for it. If the parity also spins back up at the same time, then a write is happening to that data drive.
In terms of why, the first guess is that your docker image file (or folder) is sitting on a data disk. Alternatively, your appdata share is on a data drive.
Additionally, if you're running NextCloud / Owncloud then due to their own designs, any disk which is a member of its media shares will always stay spinning.