Microsoft mouse acting erratically randomly with Thunderbolt to VGA connected? I am unaware if a third party adapter (Thunderbolt to VGA) would cause any erratic behaviour with a wired USB mouse? There is also an HDMI cable plugged in for an extended desktop. 

Any ideas? Could that be the cause.
It seems very unlikely that it should be the case. 

It might be possible if the adapter was somehow defective and sends out EMI (i.e. "noise") that is picked up by the USB cable for the mouse. If that is the case, your log should be filled with USB errors. However I do not find this scenario to be likely at all.