The O-ring found in many seal designs is a natural vibration damper. Harmonic vibration is a major source. The seal is vibrating in harmony with another piece of equipment Cavitation is very common in many applications. Remember that there are five types of cavitation and all of them can cause vibration problems. Slip stick vibration can occur between seal faces if the pumping fluid is not a lubricant.
Hot water is a good example of a non-lubricating liquid.
Most gases and dry solids are obvious non-lubricants. Hitting a critical speed is another cause of vibration. Pumps that have this problem are said to have flexible rather than rigid shafts. Out of balance rotating seal and pump components. Be aware that thermal expansion of the shaft can sometimes grab the close fitting vibration damper and open the lapped seal faces.