One possibility: Autopilot on as thrust comes up on miss. 737 tends to try to pitch up strongly. If AP is on the AP must roll in lots of nose down trim to stay engaged. If it over trimmed nose down on the go around and they turned off the AP as they were pulling power off it could have been trimmed very nose down for new condition. I've seen this but not this extreme. The airplane pitches over abruptly and its quite startling.