[EricVanWyk]

Unconnected Analog Inputs don't have a defined behavior, and should be ignored - there is nothing that defines the voltage, so you get really weird symptoms. This is not considered to be an error; putting a pull up/down resistor would have clearned up readings that don't matter but negatively affect readings that actually do matter.

What you are seeing is called "charge injection". Any muxed analog to digital converter will squirt a tiny bit of charge into or out of each channel proportional to the difference in voltage between that channel and the one before it. Since the accelerometer is the only thing active on the mux, ghosts of its signal will propogate down the chain.