Ground with one sensor vs multiple sensors
The problem is in the resistance of the cable, which causes the ground voltage of the part to fluctuate an not be at or near 0V. Sine most circuits need their ground to be at 0V.

It could have interfered with digital communication if the wire size was small enough

Since each HX711 uses 1.4mA of current resistance in the ohms range of the ground could produce voltage offsets in the ground in the mV range which could be bad for the analog electronics as well. 

Most of the time a ground should be less than 1Ω in a setup like this, a great ground should be in the 10mΩ's or less.