Each PSoC pin has two internal protection diodes, connected between Vdd and Vss. Therefore, if PSoC is not powered and signal level is applied to pin, the protection diode will conduct current when signal level is more 0.6-0.7V relatively. This current flows via Vdd pin to your board supply wire, shunting the signal source. To eliminate bus shunting, please power PSoC or disconnect bus from PSoC during production tests. It is not recommended to apply input voltage to the pin when device is not powered as it may damage the pin due to excessive current flowing through the protection diode.
Please be advised that the default drive mode of the pins is HIGH Z Analog in the reset state except P0 and P1. P1 and P1 drive resistive low during XRES for some time.