Non-magnetic low voltage transformers are essentially cheap switch-mode power supplies, and rectify the mains into a capacitor, then down convert it at a high frequency.
Trailing edge dimmers are needed to feed these capacitive loads, because the inrush current from a leading edge dimmer would stress the capacitor and rectifiers.
Trailing edge dimmers cannot use an SCR or TRIAC, and have to use a pair of MOSFETs or IGBTs which makes them more expensive.
