The possible reasons are:
1. Due to the oil temperature is too low, which increases the viscosity of oil.
2. Due to the oil flow is blocked, such as the throttle on the oil cooler is turned down, the orifice plate or needle gate of the bearing are blocked.
3. Because the auxiliary oil pump starts due to misoperation.
4. As for the system that relies on oil spill valve action to reduce oil pressure in case  the oil amount is too much, if the oil spill valve is shut down due to fault,  the oil pressure will increase in the system.
5. Due to the increasing power grid frequency, the steam turbine rotating speed rises, the high pressure oil and lubricating oil pressure will have a little increase.（js）