when we takeoff from high elevation airports such as at 3800amsl,it is often that the cabin rate descends on climb,why is this so
No different to flying along at 3800ft unpressurised and then setting up normal pressurisation.
Although it will vary between aircraft, due to design pressure differential, there will be a pressurisation schedule for the aircraft such that, for a given pressure height outside the aircraft, X, the cabin altitude will be Y.
As the aircraft climbs from SL/descends from cruise while pressurised, the cabin will climb/descend at a lesser rate according to the pressurisation schedule.
Now, if we start off somewhere other than SL or cruise - unpressurised - and then set up pressurisation, the cabin altitude first will change from the outside pressure equivalent to whatever the schedule calls for at that outside pressure.
Hence, if you takeoff from an above SL aerodrome, but lower than the maximum FL at which the pressurisation can maintain SL cabin altitude, you would expect to see the cabin descend to/towards SL as the initial setup and then climb per the schedule as the aircraft climbs.
There are plenty of atmospheric calculators on the net (the equations you can find in the appropriate textbooks) for you to play with. You can find one example
here.