I know what a ground loop is and what it is defined as and how to avoid them. I am wondering what causes the current between the two ground points to alternate. Is it the ac power to the device or maybe interference from elsewhere?
No need to define the terms: ground loop or interference or ground in your answer unless you suspect I misunderstand what it means and need to correct my understanding.
Thing is, that if two points in a circuit are actually "ground" (i.e. 0V potential) then by definition, no current can flow between these two points.
When current does flow between two arbitrary points on a circuit (that we may consider to be 'ground') this actually means that one or another of these points is actually not ground. Only one of them is. Conventional current flows from high potential to low potential.
I have had this experience working with a camera and wireless audio gear that was entirely DC powered. The 'ground loop' that was set up was only curable by cutting the ground line in the balanced line audio link back to the camera. (This only worked because the feed back to the camera was balanced). The potential difference caused the battery pinch to induce an AC (audible) signal in through the power connection - which ideally shouldn't have mattered - but it affected the wireless audio output.
A potential difference such as this would ideally not actually create anything audible, other than the fact that there was possibly a design flaw in either the power supply equipment and/or the wireless gear I was using.
I suspect that this situation is common - the undesired potential difference coupled with an equipment flaw causes the audible artifact that we detect as a 'ground loop'.
Changes in the magnetic flux through a surface induce a circular electric field. Apart from inducing current, it messes up the function of the shield as ground reference. Where does the magnetic flux come from? Neutral and power in the house usually occur in a balanced manner but it doesn't reach the house in a balanced manner since having separate and isolated ground cables would significantly increase the price of wiring. Also while currents in power wiring are balanced, the same does not hold for voltage, and changing electric fields induce magnetic fields again.
