Here's my proposal. You ready?:
What if... The less toughness you have, the more crit damage you inadvertently take. But, there's a threshold, and this threshold cannot go below 0%. Essentially, it'd be like Reckless, minus the boost in damage and it applies to everyone and ONLY in PVP, and it also adapts to world tier.
So let's say at 330k Toughness, WT5, is where the threshold is at 0%, but as your toughness drops in WT5, that threshold number increases, or how much extra crit damage you take, to 5%, 11.56%, etc.
It's pretty obvious, but you can take a guess as to what this threshold is trying to combat.