I think it's residue build up as well. Over time gelatin from the emulsion can possibly transfer over if using developer at higher temps. Also if you like to join 120 rolls with a section of tape for developing, the tape residue could come off and make that part sticky. I always use very hot water and a fine tooth brush to wash after every roll developed. After a while I will also use a bit of hot soapy water and do a more thorough cleaning.