As tubby correctly said, electrical transients.
Our SOP is to control the electrical supply. After landing and preparing for single engine taxi the FO will ask if he is clear to turn off the number two generator. The Captain is to not make any steering or braking inputs while the generator is being turned off. This allows the FO to shut down the engine after the cool down period.
As to your other point, I have occasionally experienced a bleed off in brake pressure when the number two engine is shut down. This does not happen all the time, the pressure does not go to zero, just braking effectiveness is decreased for a few seconds before it resumes to normal.
Anybody have any ideas on that?