Hmmmmmm.......
Motors run on current. Voltage "pushes" current through impedance. (yes, I know that power is the product of voltage and current at each instant, or, V*I * cos(phase angle), but the important factor is current that produces the magnetic field)
If the currents are not balanced, then the motor is not drawing full power. If it were drawing full power, about 60 to 70% of current would be in-phase power-producing (resistive) current. AND the currents would be well balanced among the phases.
My inclination when I see or hear of unbalanced currents is to disconnect ALL the "balance" capacitors, and see what the currents look like then. Balance caps can cause problems, they are not a cure-all, and more is not better..
But there is also an issue if the connection of the pump causes the breaker to open. Either there is a wrong size breaker, OR the load is way too large. Breakers will usually hold double current for a considerable time, and double current is a substantial overload...so a breaker that pops right away may be seeing a VERY large current relative to its rating.
That needs to be looked at FIRST. Something is clearly wrong, either wiring, a pump or relief valve problem, etc. A wiring issue on motor coils might cause an imbalance, although I'd expect more issues "unloaded" as well.