ADC Injection current effect

[hioyo]

The below image is from ST micro document.
The document says

" A negative injection current on any analog pin (or a closely positioned digital input pin) may introduce leakage current into the ADC input. The worst case is the adjacent analog channel. A negative injection current is introduced when VAIN < VSS, causing current to flow out from the I/O pin."

I understood how injection current is generated. But May I know how this injection current causes a leakage current in the adjacent channel.

 

[FvM]

Consider the forward biased substrate diode as base-emitter junction of a parasitic transistor, collector at the adjacent input. Current gain is possibly small (< 1) but large enough to cause unpleasant leakage currents.

The effect is also observed with 74HC40xx analog multiplexers.