Reduction of the airflow, in turn, leads to lower temperature of the indoor coil, because the amount of air moving across it is no longer sufficient to maintain the proper temperature.
This, in turn, will eventually lead to the incomplete evaporation of the refrigerant, which means that the liquid will leave the indoor coil (which it is not supposed to do) and reach the compressor. Usually, the compressors are incapable of handling liquids (with exception of "scroll" type compressors), and liquid refrigerant reaching the compressor will cause "slugging", which will eventually damage or destroy the compressor.
On the other hand, dropping temperature of the indoor coil will eventually lead to the freezing of the water vapor contained in the air on the coil surface, which will further degrade the heat transfer capability and reduce the airflow, thus triggering the domino effect.
Note: Credit for this section goes to T. Shannon Gilvary. The materials on this page are a digest from "Excessive static pressure relief" discussion thread on DIY-Zoning-general mailing list. The complete thread may be found here.
There is no cure from all diseases, and the solutions to the problem of relieving excess static pressure relief range from simple and workable, but not without pitfalls, to significantly more complicated and somewhat more expensive.
Pros: Simple, relatively inexpensive, allows additional humidity control. Avoids overshooting problems since cooled air is not dumped in areas where it is not needed.
Cons: Unmanageable, prone to mechanical failures. Improperly tuned bypass damper will either not serve the purpose (if it is too tight), or waste the energy (if it is too loose). Doesn't protect the unit from freezing or slugging - there's no feedback loop. Requires modification of ductwork.
Initial modeling shows that the threshold should be set around 30-40%. Anything less, and the static pressure increases unacceptably. Anything more, balancing quality degrades.
Pros: Simple (no hardware required at all), inexpensive (zero dollars zero cents), configurable.
Cons: Careful tuning required. Doesn't protect the unit - there's no feedback loop. This and following solutions will share the common problem: dump zones will be prone to overshooting (overcooling when cooling and overheating when heating).
Note: This solution allows to adequately solve the problem of controlling the static pressure, but there's a problem of preventing the coil freeze and slugging the compressor which is yet to be solved.
Pros: Couple of sensors are probably less expensive than the bypass damper, this solution has all the advantages of the software. Assumption is removed from the picture - the real static pressure is measured and controlled. No ductwork modification involved. Freezing and slugging is less likely but not completely eliminated.
Cons: Still no temperature feedback from the unit.
Pros: Unit condition can be determined with high probability, adding fault tolerance.
Cons: Even though the trends can be analyzed and decision made, those temperature readings do not guarantee 100% that the coil will not freeze.
Note: It is also possible to install temperature sensors not for the air stream across indoor coil, but for the refrigerant flow - one sensor immediately before the coil, the other after. It is not quite clear how to analyze the temperature patterns yet (they turned out to be quite bizarre), and how to correlate temperature readings against saturation temperature (requires further investigation), but one thing is certain: if the outgoing refrigerant temperature is getting to a water freezing point, something has to be done.
Pros: The coil will not freeze.
Cons: It is still possible to slug the compressor. Indoor coil is almost always not easily accessible. Though relatively simple, this requires the HVAC unit modification, which may void your warranty, if you're not careful enough.
The suction line is not easily accessible in a package unit. Installing a pressure sensor on the suction line is a procedure that can be performed only by a HVAC certified technician. Kids, don't do this at home. Warranty is most probably bye-bye. Check with the manufacturer of your unit.
Pros: Almost bulletproof solution.
Cons: Careful tuning is required - the saturation temperature/pressure chart will be different for different refrigerants. It's not quite clear if this is worth the trouble, possibly, it's an overkill. Remains to be seen.
Pros: The best protection and static pressure control.
Cons: Requires ductwork modification, installation of several sensors, some of them expensive, and may be overkill.