JF has got the bones of it.
the ac was capturing the 2000ft (alt*)
demanded by the fcu setting (target altitude). in alt* altitude satisfaction overrides speed target. until then the ac was flying to V2 to V2+10 (called SRS - speed
reference system).the ac would have reached 2000ft very quickly and SRS was protecting the speed until alt*. alt* occurs approx one third the roc (in feet to go) so, you can see, this all happened very rapidly.
I quote:
"the alt* mode has internal V/S guidance that is a direct function of the difference between present alt and the alt target.this
V/S guidance has priority over the target speed. therefore if a thrust reduction occurs in alt* (engine out) the V/S order will not be modified and a speed variation will occur."
the (engine out) in para above is from the book,not my add-on.
in my humble opinion,the aft cg and the trim setting would have made little difference except,perhaps,account for the initial overrotation.
best wishes to you JF.