Strong orientational effects in the static and dynamic structure factors of diblock copolymer melts have been shown to originate from the connected structure and mutual repulsion between the blocks. A collective variable based on the chain bond vectors is introduced and shown to have a dynamics comparable to that of the concentration fluctuations. Coupled equations of motion are derived which illustrate how the concentration fluctuations drive the orientational correlations in diblock chains. The coupling gives rise to two relaxation modes, one of which shows a critical slowing down and is approximately wavenumber independent on scales larger than the chain size. The amplitude of this mode increases as the microphase separation is approached in a manner similar to the static concentration fluctuations. These findings are in qualitative agreement with recent light-scattering experiments.