M609, since the slides take approx 4-6sec to deploy, it's possible (some) pax may have run past the crew (very possible if panicking) and jumped on a slide which was not fully deployed... having seen this happen in the hangar you would actually get quite a fall even with a partially inflated slide.
I wouldn't think the doors were disarmed at that point of flight (they shouldn't be for sure), more likely that the pax may have got a bit out of control... a very high possibility especially if many were not regular flyers.