It's a balancing thing, to make tryndamere's power level more consistent across different levels of play.
Making it auto-proc on fatal damage is undoubtedly a buff - you get longer out of it, and if you're running, you won't blow your ult unless it's needed. However, it also makes the ability more consistent (in this case, consistently stronger).
This means that they can nerf the duration slightly (6 secs -> 5.5 or 5 secs, something like that), without trynd losing any power.
The toggle exists so that, in situations where trynd would rather die than blow his ult, he can (simply changing it to auto-proc on fatal damage is both a nerf to the ability, and a terrible change - that's like, if blitz's mana barrer were his ult).