We have a an induction motor, fairly old, 25 years and running smoothly, but from past few years, when greasing is done, it takes around 20 days to get to normal temperature for the DE side, and sometimes sudden temperature spikes also occur, we are thinking that there might be a problem with bearing housing, but can't figure out what exactly it is.

Spikes could be due to problem with the temperature measurement system. The temperature can be high if the bearing is over greased as well. Any how the motor is very old and has to be overhauled.

i hope it could be a maintenance problem, if it is done manually. means could be on of the reasons of human error during alignment, hammering, encasing, compatibility in terms of aging, etc

How often you are greasing motor bearings ?
While greasing motor should not run. First Remove the grease drain cap at bottom of bearing housing. Slowly rotate the motor manually and inject new grease. In this process, you can observe old grease comes out from drain plug. Stop injecting grease when you observe new grease comes out from bottom drain plug. When new grease comes out from drain plug, you can ensure all old grease has drained out.

As Mr. Vijayakumar mentioned over greasing causes a bearing to heat.

It is not clear what sort of grease relief is fitted to the motor. If it is a plug that is removed for greasing then replaced you will find the the bearing is effectively over packed which is why it runs hot. Also what will be happening, is the excess grease will be pushed along the shaft inside the motor and deposited on the windings. In time this will result in a motor burn out. I would suggest that you go back to the manufacturer for advice on the correct procedure for greasing.

You have not identified if the motor is running or stopped whilst the grease is injected.