I believe you fellows have discovered a law of physics. Mechanical energy can be converted to heat energy. Trouble is, someone else beat you to it.
Excuse the ribbing. I couldn't resist.
The continouos motion of a rotating tank agitates the developer as it should. It also imparts energy to the developer. Because you have the tank in a constant temperature bath at the desired temperature, the energy has no place to go. There must be a temperature differential for it to flow out of the liquid in the tank to the liquid outside the tank. The temperature difference will necessarily be such as to reach equilibrium. Nature will see to it that there is a higher temperature in the tank so that the energy of motion can flow out. If it stays in, the temperature gets higher until it can get out.