I have no insight into Airbus'es design process on this system, but just reasoning from first principles here: The pressurization control system, like any other closed loop control system, is constantly undoing tiny disturbances to meet its target. Like balancing a ruler on your fingertip. And, when doing that, it helps to gently place it on your fingertip, in as still and upright of a state as possible... instead of suddenly throwing it on there and yelling "go!" And when the pressurization control system goes from wide open on the ground (not controlled) to controlling, it's kind of like the sudden yelling "go." There's no easy starting point.
If they tried to make it go straight for the +500 fpm after liftoff, the outflow valve does not know where to immediately go from wide open, so the only way to find the sweet spot would be to slowly close until the computer sees the +500. In the meanwhile, the cabin rate is gradually coming down from +3000 to +500, and the passengers are uncomfortable.
So, instead, while still on the roll it closes until it sees -400, and now it has a good already-stable starting point from which to gradually control to +500, (probably some time late in the roll or around the time of liftoff) and resume the climb from there, already under control.