LACK OF USE:
Compressors not run regularly will be starved for lubrication when they are turned on. This causes excessive ware until the oil again reaches the compressor. It's best to use the A/C once a week for a few minutes.

FAILURE TO FLUSH SYSTEM:
Compressors and hoses wear causing bits and pieces to mix with the oil. This sludge gets into the compressor and can cause it to seize. Failure to properly flush or using flush that leaves a residue is a primary cause of failure.

EXCESSIVE HIGH OR LOW PRESSURE:
Condenser fans can quit due to bad connections or defective motor windings that get too hot. This creates excessive head pressure and heat that can lock up or damage a compressor. Loose connections at the coil can cause high resistance and low voltage causing clutch slippage. Low refrigerant charge will cause lack of lubrication getting to the compressor. Painting condensers will insulate them from efficiently removing heat and increase head pressure in the compressor.

IMPROPER FREON OR OIL CHARGE:
Compressors manufactured after 1990 have smaller capacity, therefore use less refrigerant and oil and are extremely sensitive to inadequate amounts of oil. The wrong type of oil may break down in high heat conditions. R134a systems are more sensitive to lubrication than R12 systems. An over charge of oil can clog the condenser, the orifice tube or expansion valve and starve the compressor of oil. R12 systems use mineral oil and R134a systems use PAG or Double End Capped PAG Oil (always check the OEM specs)

REFRIGERANT BLENDS
They can affect seals and O-rings, causing them to leak, swell or otherwise deteriorate and it can happen relatively quick.