The Taser Suit shard is not proc'ing from assassins on your back. It only proc's from assassins that are already off your back who attack you or from regular monsters. 

The electrocute can however knock assassins that are on your back off, if it proc's from other monsters.

The way I tested this was for many games in the early game waiting taking only damage from the assassin on my back.. never proc'ed in many many tests. Very unlikely that it was bad luck.