Digital current affects analog signals, but why not the other way around?
[The term "digital signal" is only a convenient shorthand.  There is digital information imposed on an analog signal.]

The analog current will generate a voltage drop in the common wire too, and it will affect the digital line.  However, the drop in the common wire would have to be larger than a logic threshold in order to affect a digital signal.  That's why digital signal often is less susceptible to such interference.