I just moved traps to back day. Mine are overdeveloped thanks doing them with shoulders (just db shrugs) and then hitting them on back day with heavy deads and rows (done with my back a bit higher than parallel to the floor which brings them into play more). Four day split over six days, hitting them twice, very heavy, and your's will grow like crazy too.
I have always done shoulders with traps even though its not usually possible to hit them again on back day. I just feel that theyre more involved indirectly with my shoulders exercises. Esp. lateral raises, behind neck presses, etc. Trying to really bring up my traps to go hand in hand with how wide my shoulders are.
If you're doing them on shoulder day and then do bb rows and/or deads on back day, you're working them twice. Nothing wrong with that, but that's what's happening.