Well, the problems seems indeed to be the entry for SMPlayer.
Installed VLC and Digikam, none of them caused trouble in Device Manager's behaviour.
So what's wrong with the SMPLayer entry?
The entry's name is "Play DVD with SMPlayer".
Judging by the name, it is intended to display a popup when a DVD is inserted and let you play it with SMPlayer.
But looking at the settings, this is not the case.
The parameter The device must be of the type Storage Volume is causing the trouble.
Parameter Type: Device Interface Match
Device Type: Storage Volume
It's telling Device Manager to popup the option for SMPlayer when a storage device is plugged in regardless its content.
So, I changed the entry like this:
Parameter Type: Property Match
Device Type: Optical Disc
Value Name: Available Content
Now the option for SMPlayer popups only when an optical disc containing DVD Video is inserted.
After that change, my Device Manager works as it should.
Of course, it is easier to simply remove the SMPlayer entry (from the root account).
But if you want to have it for playing DVD's like it is intended, you could alter the entry's settings as above (again from the root account).
I don't know if this issue can be fixed from our packager or the developer of the application.