Jfill,
Judging from all the smoke and soot on the stairway of the air stair I would say that it was likely opened in an evacuation attempt. It is also possible that the impact with berm caused the door to come open like that.
Another thought I had is that perhaps the door came unlatched during the take-off roll and it was the "Door Unsafe" CAS message that triggered the abort.
It's impossible to tell from the photos whether or not the Ground Spoilers deployed, but as Mr Barrett points out if they were not not armed, the crew would not be able to get their speeds to "box" along with a blue CAS "Ground Spoiler Unarmed" CAS msg when they took the runway. It is permissible to dispatch any Gulfstream with the Ground Spoilers disabled, however it requires the use of Flaps 20 and Anti Skid functioning on a non-contaminated runway to do so. In this case the Flaps were supposedly set at 10, so that would seem to remove that possibility.
About the only thing I can deduce from the accident photos is that the elevator Trim appears to be correctly set nose up and the Thrust Reversers were not deployed, which makes me wonder if the crew pulled on the piggyback levers to command them but for some reason the buckets failed to deploy and they got forward thrust instead. I recall a Lear accident a few years ago where a tire blew on the take-off roll and pieces of the tire disabled the W.O.W. System preventing deployment of the T/R's, the aircraft subsequently went off the end of the runway at considerable speed because they thought they were commanding reverse thrust when in fact they were getting forward thrust.