Increase instead of dip in voltage in a AC to DC Motor Circuit
The motor will produce back-EMF, depending on the RPM, which effectively acts as a generator. If the motor is not under load, the voltage at the bridge output approaches the peak voltage of the AC waveform, because at the peak voltage of 120VAC RMS no power will be transferred to the motor (just enough to replace that lost by bearing friction and windage). The diodes only allow power to be transferred to the motor. 

That peak voltage is \$\sqrt{2}\cdot 120\$ = 170V approximately. 

As the motor is more heavily loaded, you would expect the voltage you read at the bridge output to drop.