Hi, 5000 trynd games veteran here:
The algorithm for determining whether you critically strike isn't random, or even pseudorandom. The actual chance that you will crit goes up if you haven't crit on your last attack, and goes down if you have. The point of that is to prevent people from making long strings of crits.
However, I believe based on personal experience that if you critical strike chance changes then the algorithm appears resets. This means Trynd can engage you with 90 fury, probably get a crit, his crit chance will change as he gets 10 more fury, and then he will have approximately a 35% chance of critting again. His two attacks would then be ~34% ish chance to crit and ~35% ish chance to crit for about a 11% chance to make two crits in a row-- not all that uncommon.
If you went in without knowing that as Trynd, building 100 fury first, you'd have a 35% chance to crit on the first attack and a severely diminished chance to crit on the next one, something like ~35% and then ~25%, yielding a much lower chance to make two crits in a row.
This sort of thing still works even as Trynd buys crit chance.
Besides that, Yasuo's crits are diminished by 10% each, and yet Tryndamere's crits are not and they are amplified by his free passive 25-60 attack damage on bloodlust. Its not uncommon for Trynd to literally hit twice as hard as Yasuo, so your crits feel less impactful.