Same thing happens to me too, everything is humming along fine, then a few yellow leaves. Sometimes, I can get it back on track immediately, sometimes it takes longer.
I think this is the real enemy behind "organic" exponential growth spurts actually. You get things really optimized and all the mixture, flora optimized and synergistic symbiosis underneath, then it gradually drifts out of those parameters, and it's almost like a cascading failure, taking away what could or should have been, lost potential, then some lost time of correcting and building it back up again.
Do you replenish your microbes though out your flowering cycles?
I find if you can ride the curve along, and keep your Ph balance just right, and keep feeding in the correct nutrients, keeping that flora proliferation rates high, keep blooming flora at root level, creates huge buds above, when the conditions are just prime. Keeping soil microbes is a chore.