Very recently I was hired to reverse engineer a commercial dimmer application. After all these years, this was the first dimmer that i found with negative power supply. To be more specific, the power supply (that was non-isolated) provided 0 and -5V for the controller. The positive of the controller chip was connected to the 0V and the negative to the -5V. Why? Because obviousely the manufacturer wanted to have a negative supply triggering for the TRIAC.
Here is a very nice document explaining these different TRIAC triggering techniques. [link]