Since ryder is freaking out so hard and the OP doesn't know whats going on.
This is a function between multiplicative buffs and cooldown times. Specifically Primal Armor and Fortress.
Far as i can tell from testing: When a multiplicative buff procs, it gives a flat stat raise. So if you proc fortress with mitigation from death up, then when mitigation from death wears off your fortress buff will retain the increase it had due to that temporary buff until it wears off.
The stacking is created by having 2 multiplicative buffs with limited duration which can be juggled, and by mighty tank linking armor to AP.
Deathproof allows you to have almost no cooldown on boulderdash, primal armor doubles your armor when boulderdash ends. Fortress multiples all of your stats by 30%.
So when fortress procs, 30% armor/dmg increase. When you boulderdash, armor is doubled based on the fortress increased value. If you let fortress wear off while primal armor remains active, then procing fortress generates a new value for the buff. Let primal armor wear off while fortress is up, reapply and get a new calculated value.
So on and so forth, stacking for however long you can keep from letting both buffs wear off at the same time. Additive buffs like mitigation from death can apply again once they are adding to multiplicative buff iterations that they haven't interacted with before. They mostly just help out with the initial ramp because 150k armor doesn't mean much once you're 26mil+. Constant multiplicative effects like "Through the Mob" and "Mighty Tank" are recalculating each time though, so they contribute heavily to the ramp.
Mighty tank is allowing AP to scale far beyond 40% because fortress is one buff. While boulderdash maintains the increased armor from fortress, the increased damage appears to remain as well. So each stacking of fortress counts as base AP, allowing mighty tanks 40% threshold to increase.
Proccing primal armor before it has warn off doesn't recalculate, it just refreshes the current buff value. This can be used to retain the buff between encounters.
The deathproof set bonus just takes the firepower increase over the top.
There may be nuance in how this actually works, but these are my findings which reliable recreate the results.
I have no reason to believe this isn't possible on any other character that can toggle fortress with another temporary multiplicative buff. I have only tested it on devastator.
EDIT: rejuvenation is in this build and increases AP. None the less, it does look like mighty tank is stacking on itself and recalculating "40%" against the existing value each time the buff toggle stacks. 5% of 63mil is 3.2mil. It lines up that the cap becomes irrelevant through the ramp.