Most probably the capacitance is hidden in your supply (I assume it is floating).
When connecting the HV, the local ground potential has to be moved to the middle point of your HV. This involves charging the capacitance from your local GND to earth - this kind of capacitance exists everywhere and isn't located in a single component.
You don't have resistors from the "+" inputs of the input amplifiers to your local circuit ground. So the charging current has to go "through" the "+" inputs of the OP-Amps - which is bad design as it won't do so as desired.
You should remove R14 and replace it by two identical resistors from the respective node to HV_M_GND. These resistors will then provide the path for the required charging current. Without these resistors, the "+" inputs will settle anywhere (depending on the leakage of the amplifier inputs), most probable outside the allowed input range of the amps. The resistors will also provide the required path of the input leakage to your local GND (HV_M_GND).