Why does the 737 auto-land trim up at around 400 feet RA?
My educated guess is that they had to find a way to give the autopilot more pitch authority for Autoland.  It's possible that the pitch feel system force the AP servo has to work against to flare the airplane, which would be more than its normal in-flight range and would also need to include some margin to apply more than the normal flaring range if necessary, is more than the servo can handle for one reason or another.  Slip clutch limits, torque load limits, maybe the operating range of the servo capstan, something like that.  Possibly it was fine for 95% of conditions but there were extreme cases uncovered in testing that had to be catered to.

So the system sets stab trim to a slower speed, which requires the AP servo to maintain a nose down load to maintain Vref.  The servo now has increased effective pitch up range because the first part of the flare action is just to stop pushing, and from there is has the normal nose-up-from-neutral range available as additional authority.

Rather than go to all the trouble to certify a new, more powerful servo, they worked out this little cheat to get the extra pitch range they needed.

Just a theory, which assumes this feature is only active during Autoland.  An alternative theory (say if the same feature works in normal non-Autoland approaches) would be they wanted a positive pitch up tendency built in if the GA button is pushed due to the airplane's behaviour somehow.