It's hard to recreate an accident from a relatively inexperienced point of view (no offense intended - as a "first crit" post, to me that qualifies as relatively inexperienced). Other folks might have seen it differently - like "did you see that guy cut in from the right side?? He just flew across the road, slammed into so-and-so, and went down!".
Even though you say you felt the pressure behind, I have a feeling that you were probably very close to being next to the guy to your left, and if you came over "slowly", the guy probably thought he had the spot.
You ever try and walk across a dark room? You hold your hands out, shuffle your feet forward, and you use known landmarks to figure out where you are. When you nudge something, unless you know you have to move a box out of the way, you ease and find a different way around the obstacle.
In bike racing it's similar. If you are easing over into a blind area (and, from what I can tell, it seems like you weren't 100% aware of what was to your left) and you feel a nudge, you should ease. If the nudge goes away, keep going. If not, then find a different route. In very, very, very dicey situations I could see justification in pushing hard, but right now, off the top of my head, I can't think of one. Okay, if the guy was crashing and using you to stay upright, then you can push back.
Generally speaking contact of any heavy sort indicates poor riding. Even relatively light contact is not good. The exception is if you are riding in extreme tight quarters and you get brushed regularly on both the left and right sides. Then the contact is more like a brush, just enough to move your jersey but not enough to put pressure on your skin.
The fact that you went down (and not the other guy, at least not so I can tell) means that you were probably at fault. It's unusual for the guy in front to go down (except Julian Dean leading out Thor, but that was a crazy tip over). I'm glad you're relatively okay, ditto your bike. You're fit so that's good, just focus a bit on the group riding thing.
hope this helps,
cdr