From the 737NG FCOM:
400 Feet Radio Altitude
The stabilizer is automatically trimmed an additional amount nose up. If the A/Ps subsequently disengage, forward control column force may be required to hold the desired pitch attitude.
If FLARE is not armed by approximately 350 feet RA, both A/Ps automatically disengage.
For the media component, you can listen to the trim wheels in this auto-land video (YouTube).
While I understand the design reason may not be public, educated guesses wrt the aerodynamics involved, and/or answers from experienced 737 pilots, are welcome.
When the plane trims up while in auto pilot, the auto pilot would push the control column forward. So what I'm thinking of, is during a go-around, somehow it's better to relieve this control column pressure for a nose-up, rather than pull back?
I checked the A320 since it is of the same category, and as far as I've looked, it looks like the A320 doesn't use that system, which tells me it may not be a certification requirement (not 100% sure).