i want to now what are main factors that may cause lube oil emulsion?

So may things!!Water and......
Type of lubricant and viscosity.(base fluid, additives. Higher viscosities form stable emmulsions more easily )
Contamination(dirt, contamination of lube oil with other lubricants and fluids etc) can cause the formation of stable emmulsions.
Aeration can look like an emmulsion before separation.
There are lubricants designed to separate from water more readily ie Turbine oils and the measurement for good seperability is the demulse value.IP 19 and ASTM D1401