Going about with likely logical deduction based off past behavior, it seems less likely that officer Wilson tried to grab Brown, a big guy, while still in his vehicle from a lower position of leverage. Physics dictate it is not possible, and it is not something any cop or anyone is trained to do. Is there a chance that Wilson tried to grab Brown while still in his vehicle based on the evidence we know 100% in this case so far? Sure there is a small chance, but I think there is a bigger chance of pigs to start flying out of my anus.
What is more likely is after Wilson slowed down to yell at the two to stop jaywalking, he probably put the two teens on edge since they had just robbed a store with physical violence and were both very high. They think they are safe as he starts to move away after yelling at them to move along. I am pretty sure those two had their sphincters pucker up there and went through a complete adrenaline dump when Wilson first slowed down. Then to see Wilson stop and start to back up to them probably sent them over the edge. More than likely Wilson backed up to them and opened his car door to get out and that is when Brown pounced to prevent Wilson from getting out. A big guy with superior leverage position thought he could get the jump on the cop and get away.
Out of those two scenarios and based on the currently known evidence, which seems like the more likely one?
We don't know near as much about Brown's personal history, but one doesn't just decide on a whim one day to smoke a bunch of weed and then rob a store with violence. Unless he just had a complete brain imbalance that day from hitting his head, or went through a traumatic experience that day, no one in human history has ever done all that together on a whim. No one starts doing major crimes that they know are crimes without working up to them with smaller crimes without extreme extenuating circumstances. It just doesn't happen. I am willing to allow that maybe something happened that day or very recently to that day which made Brown snap and go stupid with regards to running afoul of the law, but the more likely scenario was that the robbery was not his first rodeo.