Solution:

It is because we use only one A/D chip on the modules. Since we have to switch the input channel with multiplexer, if user let the channel floating, the reading will be influenced by previous channel. The simplest way to avoid the influence is connecting all the channel you need to monitor to signal, either input signal or GND. That will be OK for most case. However, if you are considering the line breaking case, there is one simple way to solve it. That is to couple a big resister (about 150K ohm) between the input signal and GND, and it will force the input to be 0V while the line is broken.