Oh yeah, you wanted real reasons.
1. The number one reason is bad planning.
2. The tank is drilled on the bottom and a riser tube is not installed. Again, bad planning...
3. The installer does not understand overflows. Again, the retard factor
4. As mentioned above, more water is allowed to drain from the tank than the wet/dry can hold. Again, bad planning...
5. A dead fish infront of the outlet when a secondary outlet was neglected because "that'll never happen". Again, bad planning...
6. An overflow that was improperly designed and never tested to see if it will restart after a power failure. Enter the retard factor one more time...
7. A properly designed overflow that does not have a ball valve to restrict the flow, and then later modified by plugging or capping the vent because it is "too noisy". (Retard factor).
8. The wet/dry is later filled to run like a sump and then the power goes out.
9. To lessen the number of times the system needs to be topped off, extra water is added reducing the capacity to hold the emergency surge volume.
10. OOPS!