My guess would be that FCPX, upon splitting the stereo pair, pans both mono files center. Depending on where your original audio levels are (and where are they, and how much are you increasing them?), they are somewhat "doubling" the sound level since it's two copies of the same source. That equals a jump of approximately +3dB.
Your dialog levels should not be that loud. They should be recorded, and mixed, to average between -20dB and -12dB.
Your dialog levels should not be that loud. They should be recorded, and mixed, to average between -20dB and -12dB.