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D8 when forward biassed has a maximum voltage drop of 0.875 volts @3A. Therefore you could get an input voltage of 5.875 volts on the A to D input pin. The absolute maximum voltage for any input pin is 5.5VDC.
When you are dealing with high frequency induced spikes you really have to be carefull with A to D inputs. Another option is to use a ferrite on the cable to help attenuate any spikes.
The probable reason that the LEM300 is causing it more than the LEM 500 is that at high currents the output of the LEM300 is pretty close to the 5 volt point so any induced spike more easily pushes it over the max voltage.
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