back to Boeing:
Re,
Just an educated guess.
But no slides deployed so decided not to evacuate. Decision made to disembark via rear doors. So whilst there had been an "incident" the crew decided to action the appropriate after landing scan/procedure. Also this would allow any fire services better access to wheel/engine areas.
Hence flaps retracted and thus why flaps not in the 40 position.
I realize that's just a guess.
(Not sure of your experience but,) landing flap is either 30 or 40. Normally one leaves things as they are in such events. There are no statements in the FCOM or QRH that require changing configuration (for the reasons offered). The B737 FCOM Evacuation checklist states that the Captain sets the park brake and the F/O extends the flaps to 40, etc., (i.e., nothing about retracting them again).
ed to add:
JACDEC(linked previously by another poster), provides an unverified statement indicating:
Flight phase: LDG – Flare, Landing Roll, Aborted Landing after Touchdown
The recorders will tell us soon enough about flap position, reverse use, (there is no high engine noise on the video of the landing I heard), stability of the approach, approach speed, touchdown point, tailwind component, etc.