What are the most common causes for the lapped seal faces to open?

High temperature may effect.  

Due to high stuffing box pressure.

Some cases, high shaft speed.

The face load might have unbalanced seal designs

Also cavitations causes problem

Some cases, high shaft speed The face load might have unbalanced seal designs

there for High temperature may effect

Major causes of seal face opening are

1. Improper lapping of seal faces causing failure of thin film.

2. High shaft speed causing whirl in shaft.

3. High temparature.

4. In rotating head assembly springs not bearing the load properly because of precipitation of working fluid.

5. Incorrect choice of stationary or rotating face.

i think running the machine under over vibration circumstances and exceeding the allowable vibration of it

whirl in shaft

improper lapping

high temperature

low viscosity on lubricating oil