AFAIK, the idea of a pH crash was around long before we started adding CO2 to the water. In that case, I belive what happened was either the plants consumed all the carbonates, thus removing the buffer and allowing naturally produced acids from decomp and fish waste to lower the pH to dangerous levels, or... lack of water changes caused organic acids and natural biological process to slowly remove carbonates, thus removing the buffer and allowing pH to get very low.
I have had this happen on a small 2.5 gallon shrimp tank. I neglected it for a long time and then the shrimp died. It was planted with a bunch of anacharis, which eventually consumed the carbonates and pH was like 5 when I checked it.
Now, as for our typical situation, to the best of my knowledge your only issue with low/no buffer is when using a pH controller to inject CO2, as was already described.
Long story short, could happen with a pH controller and CO2, or could happen with really, really, really bad tank maintenance.