It's probably a capacitive touch sensor relying on a ground reference. If the machine is not grounded when you touch it, it will just remain floating and have no potential difference to trigger the sensor. (you also create a potential by connecting yourself to the DC common point). If it's just a 2 pin AC transformer, it'll be relying on something like inter-winding capacitance (or have some sort of Y cap across it, unsure, haven't had one of these to examine).
Other capacitive touch sensors such as those for screens get around this by referencing to themselves through creating a tiny AC field feeding back on itself that gets disturbed when contacted, and can electronically filter out anything not a finger or stylus that recreates that feild distortion, basically like a metal detector except with capacitance instead of inductance.